We can have --no-squash update the --squash state directly so we don't
have to constantly check two different knobs.  This means opts.squash
is now a tristate: None means load config settings while True/False
means the user has specified a command line option.  This also means
we don't have to detect multiple --squash/--no-squash flags as the
argparse module will take care of coalescing.

Bug: 993518
Test: `git cl upload` uses my ~/.gitconfig
Test: `git cl upload --squash` squashes
Test: `git cl upload --no-squash` doesn't squash
Change-Id: Ib2577db11d3af03c93cb47d8b78b341213720a92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1754234Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>