User may have set fetch only on the old default branch. This ensures
that fetch works with new default branch.

R=gavinmak@google.com

Bug: 1268775
Change-Id: I971cdd041e8e8a7f06a20f4de348c1395c7b6e06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3274108
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>