Make ChangeLog more condensed

Just duplicating the git log isn't particularly exciting. Make
the ChangeLog a list of change subjects grouped by tag.

New output can be seen here: http://paste.openstack.org/show/49999

Change-Id: I7bf4d89dcdba135de663d75895b30963f7a44662
Let git do mailmap mapping for us

It turns out, git log already knows how to process mailmap files.
Just defer to it and stop processing that in python.

Change-Id: Ib3d6f85683fcbe8b5002117d2ee46dc3c008cae7
Merge "Make ChangeLog more condensed"
Merge "Let git do mailmap mapping for us"