Allow to pass git_ref property alone.

This is required to support luci retry functionality from the
flutter-dashboard.

Change-Id: I1c4defe834925557ac4840526199d9829b7776e9
Bug: https://github.com/flutter/flutter/issues/54873
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/4480
Reviewed-by: Tong Wu <wutong@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>