command: add a `repo help` tip to --help output

For people used to running `repo xxx --help`, they might not realize
that there are detailed man pages behind `repo help xxx`.  Add a note
to all --help commands to improve discoverability.

Change-Id: I84af58aa0514cc7ead185f6c2534a8f88e09a236
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255853
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

command.py

@@ -66,7 +66,8 @@ class Command(object):
         usage = self.helpUsage.strip().replace('%prog', me)
       except AttributeError:
         usage = 'repo %s' % self.NAME
-      self._optparse = optparse.OptionParser(usage=usage)
+      epilog = 'Run `repo help %s` to view the detailed manual.' % self.NAME
+      self._optparse = optparse.OptionParser(usage=usage, epilog=epilog)
       self._Options(self._optparse)
     return self._optparse