tests: missing refs/ in bare repositories (#28844) (#28852)

Backport #28844 by @AdamMajer

Git 2.43.0 will not detect a git repository as valid without refs/
subdirectory present. `git gc` cleans this up and puts it in
packed-refs. We must keep refs/ non-empty.

Co-authored-by: Adam Majer <[email protected]>

Author: GiteaBot

tests/gitea-repositories-meta/user2/test_commit_revert.git/refs/heads/main

@@ -0,0 +1 @@
+deebcbc752e540bab4ce3ee713d3fc8fdc35b2f7