If remote HEAD returns legacy default branch, refresh HEAD by running
set-head to check if there are any changes.

R=gavinmak@google.com

Change-Id: I5891a899b512e81fccfb086a056d497c906df4b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3152593
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>