subcmds: force consistent help text format

We're inconsistent with help text as to whether it uses title case and
whether it ends in a period.  Add a test to enforce a standard, and use
the style that Python optparse & argparse use themselves (e.g. with the
--help option): always lowercase, and never trailing period.

Change-Id: Ic1defae23daeac0ac9116aaf487427f50b34050d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305144
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/diff.py

@@ -33,7 +33,7 @@ to the Unix 'patch' command.
   def _Options(self, p):
     p.add_option('-u', '--absolute',
                  dest='absolute', action='store_true',
-                 help='Paths are relative to the repository root')
+                 help='paths are relative to the repository root')
 
   def _ExecuteOne(self, absolute, project):
     """Obtains the diff for a specific project.