[Review 3-5] ChangeId support for commit-msg hooks

[Norbert Thiebaud]

The 2 following patch add the support for ChangeId and tweak it to
tolerate git-am -s (issue with extraneous empty line)

http://cgit.freedesktop.org/libreoffice/core/commit/?id=18118cd9c581659fa6f9aef1e1729e87c9fd1bba
http://cgit.freedesktop.org/libreoffice/core/commit/?id=298fbdc42af781cdd47d2cbedd945ec8c4a564bc

It might prove convenient to have them back-ported to 3-5.
This does not impact the code in any way, so there is no risk of
regression due to these patches.

Norbert