Allows builders to do a force refresh of the git cache to ensure a new
fetch happens. android-binary-size needs this to get an accurate diff
of the passed in src revision and the applied patch:
https://bugs.chromium.org/p/chromium/issues/detail?id=1129235#c19

Bug: 1129235
Change-Id: I993521cdde2a491fc9d42d75ff28dedcd2cea64e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2438802
Commit-Queue: Stephanie Kim <kimstephanie@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>