When initializing git_cache repository, we can't assume the default
branch is main. This results in HEAD pointing to invalid ref for repos
that don't have main branch.

Instead, fetch information from remote git repository.

R=gavinmak@google.com

Fixed: 1334889
Change-Id: I84e6d8e09c81bdae7e5e31af88973534431824e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3696906Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>