git2-rs errors with "no callback set" message when using git2::Cred::ssh_key_from_agent
with passworded ssh key
(works fine with passwordless ssh key)

see main.rs for minimal repro code

/*
 * With NO panic!(...) in cb.credentials callback,
 * and fetch_with_remoteconn = true;
thread 'main' panicked at src/main.rs:33:61:
called `Result::unwrap()` on an `Err` value: Error { code: -16, klass: 23, message: "authentication required but no callback set" }
stack backtrace:
   0: rust_begin_unwind
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panicking.rs:662:5
   1: core::panicking::panic_fmt
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/panicking.rs:74:14
   2: core::result::unwrap_failed
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/result.rs:1679:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/result.rs:1102:23
   4: git2_ssh_agent_repro::main
             at ./src/main.rs:32:54
   5: core::ops::function::FnOnce::call_once
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/ops/function.rs:250:5
 */

/*
 * With NO panic!(...) in cb.credentials callback,
 * and fetch_with_remoteconn = false;
fetching with Repository.remote() and callbacks.credentials
thread 'main' panicked at src/main.rs:39:65:
called `Result::unwrap()` on an `Err` value: Error { code: -16, klass: 23, message: "authentication required but no callback set" }
stack backtrace:
   0: rust_begin_unwind
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panicking.rs:662:5
   1: core::panicking::panic_fmt
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/panicking.rs:74:14
   2: core::result::unwrap_failed
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/result.rs:1679:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/result.rs:1102:23
   4: git2_ssh_agent_repro::main
             at ./src/main.rs:39:17
   5: core::ops::function::FnOnce::call_once
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/ops/function.rs:250:5
 */

/*
 * With panic!(...) in cb.credentials callback,
 * and fetch_with_remoteconn = true;
 *
thread 'main' panicked at src/main.rs:17:17:
credentials callback called
stack backtrace:
   0: rust_begin_unwind
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panicking.rs:662:5
   1: core::panicking::panic_fmt
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/panicking.rs:74:14
   2: git2_ssh_agent_repro::main::{{closure}}
             at ./src/main.rs:17:17
   3: <alloc::boxed::Box<F,A> as core::ops::function::FnMut<Args>>::call_mut
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/alloc/src/boxed.rs:2157:9
   4: git2::remote_callbacks::credentials_cb::{{closure}}
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/git2-0.19.0/src/remote_callbacks.rs:333:13
   5: std::panicking::try::do_call
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panicking.rs:554:40
   6: ___rust_try
   7: std::panicking::try
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panicking.rs:518:19
   8: std::panic::catch_unwind
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panic.rs:345:14
   9: git2::panic::wrap
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/git2-0.19.0/src/panic.rs:13:11
  10: git2::remote_callbacks::credentials_cb
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/git2-0.19.0/src/remote_callbacks.rs:315:18
  11: request_creds
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/ssh_libssh2.c:394:11
  12: _git_ssh_setup_conn
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/ssh_libssh2.c:828:16
  13: ssh_uploadpack_ls
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/ssh_libssh2.c:916:9
  14: _ssh_action
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/ssh_libssh2.c:972:11
  15: git_smart__connect
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/smart.c:164:15
  16: git_remote_connect_ext
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/remote.c:963:15
  17: git_remote_connect
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/remote.c:1002:9
  18: git2::remote::Remote::connect_auth
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/git2-0.19.0/src/call.rs:13:39
  19: git2_ssh_agent_repro::main
             at ./src/main.rs:32:54
  20: core::ops::function::FnOnce::call_once
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/ops/function.rs:250:5
 */

/*
 * With panic!(...) in cb.credentials callback,
 * and fetch_with_remoteconn = false;
fetching with Repository.remote() and callbacks.credentials
thread 'main' panicked at src/main.rs:17:17:
credentials callback called
stack backtrace:
   0: rust_begin_unwind
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panicking.rs:662:5
   1: core::panicking::panic_fmt
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/panicking.rs:74:14
   2: git2_ssh_agent_repro::main::{{closure}}
             at ./src/main.rs:17:17
   3: <alloc::boxed::Box<F,A> as core::ops::function::FnMut<Args>>::call_mut
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/alloc/src/boxed.rs:2157:9
   4: git2::remote_callbacks::credentials_cb::{{closure}}
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/git2-0.19.0/src/remote_callbacks.rs:333:13
   5: std::panicking::try::do_call
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panicking.rs:554:40
   6: ___rust_try
   7: std::panicking::try
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panicking.rs:518:19
   8: std::panic::catch_unwind
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/std/src/panic.rs:345:14
   9: git2::panic::wrap
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/git2-0.19.0/src/panic.rs:13:11
  10: git2::remote_callbacks::credentials_cb
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/git2-0.19.0/src/remote_callbacks.rs:315:18
  11: request_creds
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/ssh_libssh2.c:394:11
  12: _git_ssh_setup_conn
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/ssh_libssh2.c:828:16
  13: ssh_uploadpack_ls
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/ssh_libssh2.c:916:9
  14: _ssh_action
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/ssh_libssh2.c:972:11
  15: git_smart__connect
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/transports/smart.c:164:15
  16: git_remote_connect_ext
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/remote.c:963:15
  17: connect_or_reset_options
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/remote.c:1254:10
  18: git_remote_fetch
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libgit2-sys-0.17.0+1.8.1/libgit2/src/libgit2/remote.c:1375:15
  19: git2::remote::Remote::fetch
             at /Users/mx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/git2-0.19.0/src/call.rs:13:39
  20: git2_ssh_agent_repro::main
             at ./src/main.rs:39:17
  21: core::ops::function::FnOnce::call_once
             at /rustc/730d5d4095a264ef5f7c0a0781eea68c15431d45/library/core/src/ops/function.rs:250:5
 */