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Trying to use --help with grit just ends in errors like:
  $ grit --help
  getopt.GetoptError: option --help not recognized

Add support for it with `grit --help`.  This doesn't update the
subtools for now as that'll take much more work/thought.

We also don't support the -h short option as it's already used for
other things, and we don't want to complicate/break it.

Bug: 747171
Change-Id: Ied24f810ed583d464dc2970c8e472784e22ec044
Reviewed-on: https://chromium-review.googlesource.com/c/1327421Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608196}