Automatically install Gerrit Code Review's commit-msg hook

Most users of repo are also using Gerrit Code Review, and will want
the commit-msg hook to be automatically installed into their local
projects so that Change-Ids are assigned when commits are created,
not when they are first uploaded.

Change-Id: Ide42e93b068832f099d68a79c2863d22145d05ad
Signed-off-by: Shawn O. Pearce <sop@google.com>