Any time the git index changes, we will re-run the commit id hook.
This will ensure we never are left with stale commit ids from
other git commits.

BUG=angle:626

Change-Id: I03c37225fb902b6c1801e8f993d767122e8bfeb6
Reviewed-on: https://chromium-review.googlesource.com/197723Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>