Fix Debian GitHub build & zstd CMake error (#200)

Summary:
1. Workaround for Debian Docker image bug that is breaking Debian build on GitHub (Explicitly mark Git repo as safe).
2. Pin zstd to a commit that resolves problems with older CMakes (note: affects all OSes, not just Debian)

Context for 1: In latest Debian Docker image , there is a regression that affects the checkout action.

From actions/checkout#1169:
> - Checkout runs, and runs /usr/bin/git config --global --add safe.directory <path>
> - The global .gitconfig does not exist
> - Any calls to git remain unsafe/dubious

The suggested workaround was to use --system instead of --global.

Pull Request resolved: #200

Test Plan: See if GitHub Action Debian build is fixed.

Reviewed By: therealgymmy

Differential Revision: D43720363

Pulled By: jaesoo-fb

fbshipit-source-id: 54f3586cc7f8e72045e60d8dd454c7a77725e6b2

.github/workflows/build-cachelib-debian-10.yml

@@ -52,6 +52,9 @@ jobs:
           g++ - || true
       - name: "checkout sources"
         uses: actions/checkout@v2
+      - name: "Add Git safe directory"
+        # Workaround for Docker image bug (GitHub issue #199).
+        run: git config --system --add safe.directory $GITHUB_WORKSPACE
       - name: "Install Prerequisites"
         run: ./contrib/build.sh -S -B
       - name: "Test: update-submodules"

contrib/build-package.sh

@@ -102,11 +102,12 @@ test "$#" -eq 0 \
     && die "missing dependancy name to build. See -h for help"
 
 ######################################
-## Check which dependecy was requested
+## Check which dependency was requested
 ######################################
 
 external_git_clone=
 external_git_branch=
+# external_git_tag can also be used for commit hashes
 external_git_tag=
 update_submodules=
 cmake_custom_params=
@@ -175,7 +176,10 @@ case "$1" in
     REPODIR=cachelib/external/$NAME
     SRCDIR=$REPODIR/build/cmake
     external_git_clone=yes
-    external_git_branch=release
+    # Previously, we pinned to release branch. v1.5.4 needed
+    # CMake >= 3.18, later reverted. While waiting for v1.5.5,
+    # pin to the fix: https://github.com/facebook/zstd/pull/3510
+    external_git_tag=8420502e
     if test "$build_tests" = "yes" ; then
         cmake_custom_params="-DZSTD_BUILD_TESTS=ON"
     else