git-gui: Add option to wrap overlong lines in the commit message field.

[FeepingCreature]

If a source hoster renders commit messages with lines as paragraphs, newlines can be visually relevant. Then to avoid pointless paragraph breaks, I have to write very long commit lines. To make this easier, add gui.commitmsgwrap to set the commit message window to linewrap on words.

git-gui.pot ended up unreasonably large in the diff. I'm not sure how to avoid that?

(To be clear, in contrast to #7 , I specifically want a soft wrap. Seeing #7, maybe these features are in contradiction?)