git-gui: Flip commit message buffer and diff area.

git-gui: Flip commit message buffer and diff area.

Since Tk will only supply new space gained from growing the top level to
the bottom/right most widget within a panedwindow and most users will be
growing a git-gui main window for the purposes of seeing more of the
currently shown diff, flipping the order around makes Tk do what the
user wants by default.

Of course because we also removed the paned window from the commit buffer
area it is now impossible to increase the visible space for the commit
message.  But I don't see this as a huge concern right now as its actually
very awkward to try and balance three paned window dividers within the
same top level window.  We could always add it back if users really want
to expand the commit buffer and see more.

I also corrected a number of bugs that I accidentally introduced in the
last commit.

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