[PATCH] contrib: fix check-commit to not reject commits from `hg sign` and `hg tag`

[Sean Farley]

Augie Fackler <raf at durin42.com> writes:

> # HG changeset patch
> # User Augie Fackler <augie at google.com>
> # Date 1484800475 18000
> #      Wed Jan 18 23:34:35 2017 -0500
> # Branch stable
> # Node ID 416cd976ac56e2915e5ee7422d9b97f0d41b8e04
> # Parent  41d220e2bed95664c335f6a7ef70b8ce06dca86c
> contrib: fix check-commit to not reject commits from `hg sign` and `hg tag`
>
> I'm tired of having a spurious red build every time we do a
> release. Fix it once and for all.

Aha, I was about to write this patch just now! Looks good to me.