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: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -4126,6 +4126,7 @@ proc do_quit {} {
 	if {$is_quitting} return
 	set is_quitting 1
 
+	if {[winfo exists $ui_comm]} {
 		# -- Stash our current commit buffer.
 		#
 		set save [gitdir GITGUI_MSG]
@@ -4155,6 +4156,7 @@ proc do_quit {} {
 		if {$cfg_geometry ne $rc_geometry} {
 			catch {exec git repo-config gui.geometry $cfg_geometry}
 		}
+	}
 
 	destroy .
 }