Remove --use-mailmap as it's not needed

--use-mailmap is a newer git construct that applies mailmaps to
things that are not using the mailmap aware format strings. But
we're using those format strings, and this breaks on precise.

Simplest thing - don't use the arg, since it's pointless and broken
for us.

Change-Id: Ic8ca062a6936a4f112aa52d0eb209211658611c2