It is occasionally useful to be able to run git cl format against some
specific upstream without having to configure branches in git. As
CMDformat can't easily take "git-diff-like" args, add an explict
--upstream argument.

This has come up a few times in the past and hopefuly the new arg will
be discoverable enough in --help for git cl format to help people.

Bug: 832295, 1082520
Change-Id: I2f3a33d719104ffd97cfa388178e38bfe8731c0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3124674Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Tomasz Śniatowski <tsniatowski@vewd.com>