gclient executes `git rev-parse <ref>` to check if <ref>
already exists in the local clone, which succeeds when
<ref> is a git revision hash, even if it doesn't exist.

We execute `git rev-parse <ref>^{commit}` instead, which only
succeeds when <ref> resolves to a valid commit in the local
repo.

Change-Id: Ia1cdf7162c4c82a7e4214c6af375c1a9522d7b07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2095745Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>