sync: only use --cruft when git supports it.
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 <a6e326c88a51217312e91af047fa468d2d7dea8f@google.com>
Tested-by: LaMont Jones <01939f9990a33aa2509cfaed21d2f9ba201a2b37@google.com>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>

lewixliu/git-repo,GerritCodeReview/git-repo,GerritCodeReview/git-repo,lewixliu/git-repo