Item11267: Add a git commit-msg hook

This file can be installed to a local .git/hooks directory.

It performs commit message validations locally, to hopefully prevent
them from being rejected once the git svn rebase is attempted.

 - Checks that the first line starts with an ItemNNNN: number
 - Verifies that each item exists, and is in a state to accept checkins

This depends upon a helper topic:
  http://foswiki.org/Tasks/ItemStatusQuery?item=ItemNNNN

git-svn-id: http://svn.foswiki.org/trunk@17377 0b4bb1d4-4e5a-0410-9cc4-b2b747904278