git cl presubmit: Add "base branch" usage hint

A base branch can be passed into `git cl presubmit` as a positional
argument, but the output for `git cl presubmit --help` did not mention
it until this CL.

Change-Id: I05d75af4fd175585a5e7ae74f2f4bb73227127ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4778138
Commit-Queue: Michael Cui <mlcui@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

git_cl.py

@@ -4432,6 +4432,7 @@ def CMDlint(parser, args):
 
 
 @metrics.collector.collect_metrics('git cl presubmit')
+@subcommand.usage('[base branch]')
 def CMDpresubmit(parser, args):
   """Runs presubmit tests on the current changelist."""
   parser.add_option('-u', '--upload', action='store_true',