git-p4: add --no-verify option

git-p4: add --no-verify option

Add new command line option --no-verify:

Add a new command line option "--no-verify" to the Submit command of
git-p4.py.  This option will function in the spirit of the existing
--no-verify command line option found in git commit. It will cause the
P4 Submit function to ignore the existing p4-pre-submit.

Change the execution of the existing trigger p4-pre-submit to honor the
--no-verify option. Before exiting on failure of this hook, display
text to the user explaining which hook has failed and the impact
of using the --no-verify option.

Change the call of the p4-pre-submit hook to use the new run_git_hook
function. This is in preparation of additional hooks to be added.

Signed-off-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>