grit: delay "help" builtin parsing

The "help" ad-hoc parsing only works in the simplest case:
  grit help
  grit help foo

When passing options to grit, the "help" builtin fails and producing
confusing output:
  $ grit -i ... help
  No such tool.  Try running 'grit help' for a list of tools.
  $ grit -i ... help xmb
  No such tool.  Try running 'grit help' for a list of tools.

Delay parsing of the "help" tool until after we've processed grit's
core options.

Bug: 747171
Change-Id: Iae5fdee2493346cee4272368ac8862647433faaf
Reviewed-on: https://chromium-review.googlesource.com/c/1320690
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605854}
1 file changed