presubmit_support: Use single quotes and convert to ArgumentParser.

Change-Id: I54f0fe3a2257166cba2a69659e5128e4ac9310bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2001707Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

tests/presubmit_unittest.py

@@ -897,8 +897,7 @@ def CheckChangeOnCommit(input_api, output_api):
 
     self.assertEqual(
         sys.stderr.getvalue(),
-        'Usage: presubmit_unittest.py [options] <files...>\n'
-        '\n'
+        'usage: presubmit_unittest.py [options] <files...>\n'
         'presubmit_unittest.py: error: For unversioned directory, <files> is '
         'not optional.\n')