git-cl: simplify squash commandline parsing

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/+/1754234
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>

git_cl.py

@@ -2652,14 +2652,9 @@ class _GerritChangelistImpl(_ChangelistCodereviewBase):
 
   def CMDUploadChange(self, options, git_diff_args, custom_cl_base, change):
     """Upload the current branch to Gerrit."""
-    if options.squash and options.no_squash:
-      DieWithError('Can only use one of --squash or --no-squash')
-
-    if not options.squash and not options.no_squash:
+    if options.squash is None:
       # Load default for user, repo, squash=true, in this order.
       options.squash = settings.GetSquashGerritUploads()
-    elif options.no_squash:
-      options.squash = False
 
     remote, remote_branch = self.GetRemoteBranch()
     branch = GetTargetRef(remote, remote_branch, options.target_branch)
@@ -4740,7 +4735,7 @@ def CMDupload(parser, args):
                          'Default: remote branch head, or master')
   parser.add_option('--squash', action='store_true',
                     help='Squash multiple commits into one')
-  parser.add_option('--no-squash', action='store_true',
+  parser.add_option('--no-squash', action='store_false', dest='squash',
                     help='Don\'t squash multiple commits into one')
   parser.add_option('--topic', default=None,
                     help='Topic to specify when uploading')