git-gui: Optionally save commit buffer on exit.

git-gui: Optionally save commit buffer on exit.

If the commit area does not exist, don't save the commit message to
a file, or the window geometry.  The reason I'm doing this is I want
to make the main window entirely optional, such as if the user has
asked us to show a blame from the command line.  In such cases the
commit area won't exist and trying to get its text would cause an
error.

If we are running without the commit message area, we cannot save
our window geometry either, as the root window '.' won't be a normal
commit window.

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