git-gui: Make commit log messages end with a newline

git-gui: Make commit log messages end with a newline

Concatenating commit log messages from multiple commits works better
when all of the commits end with a clean line break.

Its good to be strict in what you create, and lenient in what you
accept, and since we're creating here, we should always try to
Do The Right Thing(tm).

Signed-off-by: Bernt Hansen <bernt@alumni.uwaterloo.ca>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>