fetchTree/fetchGit: re-enable shallow fetching

Add several tests for git fetching:
- shallow-cache-separation: can fetch the same repo shallowly and non-shallowly
- shallow-ignore-ref: ensure that ref gets ignored when shallow=true is set
- ssh-shallow: can fetch a git repo via ssh using shallow=1

tests/nixos/fetch-git/testsupport/gitea-repo.nix

@@ -72,4 +72,4 @@ in
           """)
     '';
   };
-}
+}