Treat refs/heads/main as default branch

Cr-Commit-Position will be updated in coming weeks to reflect actual
branch name. This change adds support for both old and new default
branch name.

Bug: 1236756
Change-Id: Ic8cd3a1a4905899752611cb521b5cedd31649d8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3072899


Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#908674}