HEAD may be different than what git cache has. This change ensures the
correct revision is used.

R=gavinmak@google.com

Bug: 1251783
Change-Id: I4909a4d52ef70ee806c285f24693eca5173003d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3174363
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>