scm.GIT functions are tested, and using them makes it easier to mock
git calls on git-cl tests.

Bug: 1051631
Change-Id: If067aa3f71b7478cafd7985d3020dacc0c6a41c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2055464Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>