branch list: more detailed message describing deleted branches

Fixes #1537

src/commands/branch.rs

@@ -334,10 +334,14 @@ fn cmd_branch_list(
         write!(formatter.labeled("branch"), "{name}")?;
         print_branch_target(formatter, branch_target.local_target.as_ref())?;
 
+        let mut found_non_git_remote = false;
         for (remote, remote_target) in branch_target.remote_targets.iter() {
             if Some(remote_target) == branch_target.local_target.as_ref() {
                 continue;
             }
+            if remote != "git" {
+                found_non_git_remote = true;
+            }
             write!(formatter, "  ")?;
             write!(formatter.labeled("branch"), "@{remote}")?;
             if let Some(local_target) = branch_target.local_target.as_ref() {
@@ -361,6 +365,13 @@ fn cmd_branch_list(
             }
             print_branch_target(formatter, Some(remote_target))?;
         }
+        if found_non_git_remote && branch_target.local_target.is_none() {
+            writeln!(
+                formatter,
+                "  (this branch will be *deleted permanently* on the remote on the\n   next `jj \
+                 git push`. Use `jj branch forget` to prevent this)"
+            )?;
+        }
     }
 
     Ok(())

tests/test_branch_command.rs

@@ -310,6 +310,8 @@ fn test_branch_forget_deleted_or_nonexistent_branch() {
     insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @r###"
     feature1 (deleted)
       @origin: 9f01a0e04879 message
+      (this branch will be *deleted permanently* on the remote on the
+       next `jj git push`. Use `jj branch forget` to prevent this)
     "###);
 
     // ============ End of test setup ============

tests/test_git_fetch.rs

@@ -949,5 +949,7 @@ fn test_git_fetch_remote_only_branch() {
     feature1: 9f01a0e04879 message
     feature2 (deleted)
       @origin: 9f01a0e04879 message
+      (this branch will be *deleted permanently* on the remote on the
+       next `jj git push`. Use `jj branch forget` to prevent this)
     "###);
 }

tests/test_git_push.rs

@@ -184,6 +184,8 @@ fn test_git_push_multiple() {
     insta::assert_snapshot!(stdout, @r###"
     branch1 (deleted)
       @origin: 45a3aa29e907 description 1
+      (this branch will be *deleted permanently* on the remote on the
+       next `jj git push`. Use `jj branch forget` to prevent this)
     branch2: 15dcdaa4f12f foo
       @origin (ahead by 1 commits, behind by 1 commits): 8476341eb395 description 2
     my-branch: 15dcdaa4f12f foo