Repositories may have different default branches. This ensures to fetch
remote HEAD branch which is default branch. Only downside to this
approach is that actual branch (e.g. origin/main) reference won't be
updated. Since we don't need it for this particular case, it should be
fine to use HEAD.

R=apolito@google.com, ehmaldonaldo@chromium.org

Bug: 1155492
Recipe-Nontrivial-Roll: chromiumos
Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Change-Id: Ib2e6e50901a0a10ed647acea3aeb517b83567702
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2580190Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>