It's sometimes possible to end up with a depot_tools checkout that isn't
a git repository, which breaks the update mechanism. Rather than
allowing the updater to silently fail and have things break several
weeks/months later, we should emit a warning.

Bug: 1333997
Change-Id: I70ae4f3b479e24b87987847927184687caf08b33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3692864Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Will Cassella <cassew@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>