git-gui: Cleanup end-of-line whitespace in commit messages.

git-gui: Cleanup end-of-line whitespace in commit messages.

When committing changes its useless to have trailing whitespace on the
end of a line within the commit message itself; this serves no purpose
beyond wasting space in the repository.  But it happens a lot on my
Mac OS X system if I copy text out of a Terminal.app window and paste
it into git-gui.

We now clip any trailing whitespace from the commit buffer when loading
it from a file, when saving it out to our backup file, or when making
the actual commit object.

I also fixed a bug where we lost the commit message buffer if you quit
without editing the text region.  This can happen if you quit and restart
git-gui frequently in the middle of an editing session.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>