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>