Tests should start to rely on fake_repos.DEFAULT_BRANCH so it can be
renamed in the future. This is a quick fix in case
git.init.defaultBranch is set to a custom value.

Bug: 1144918
Change-Id: I1c8cd591bee6b5fe14a5de4683111a89224cb2b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2521655
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>