FTFY: support wordwrapping commit messages

It's common for commit messages to be wrapped at odd places. git-gui
is often to blame. Adds support for automatically fixing up these
messages if running ftfy --amend, and adds a new option --msg-only for
fixing only the commit message.

Change-Id: Ia7ea529f8cb7395d34d9b39f1192598e9a1e315b