sync: only use --cruft when git supports it.

git gc --cruft was added in 2.37.0.

Bug: https://crbug.com/gerrit/16270
Change-Id: I71e46741e33472a92f16d6f11c51a23e1e55d869
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/346577


Reviewed-by: Emily Shaffer <emilyshaffer@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>