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

@@ -4126,34 +4126,36 @@ proc do_quit {} {
 	if {$is_quitting} return
 	set is_quitting 1
 
-	# -- Stash our current commit buffer.
-	#
-	set save [gitdir GITGUI_MSG]
-	set msg [string trim [$ui_comm get 0.0 end]]
-	regsub -all -line {[ \r\t]+$} $msg {} msg
-	if {(![string match amend* $commit_type]
-		|| [$ui_comm edit modified])
-		&& $msg ne {}} {
-		catch {
-			set fd [open $save w]
-			puts -nonewline $fd $msg
-			close $fd
+	if {[winfo exists $ui_comm]} {
+		# -- Stash our current commit buffer.
+		#
+		set save [gitdir GITGUI_MSG]
+		set msg [string trim [$ui_comm get 0.0 end]]
+		regsub -all -line {[ \r\t]+$} $msg {} msg
+		if {(![string match amend* $commit_type]
+			|| [$ui_comm edit modified])
+			&& $msg ne {}} {
+			catch {
+				set fd [open $save w]
+				puts -nonewline $fd $msg
+				close $fd
+			}
+		} else {
+			catch {file delete $save}
 		}
-	} else {
-		catch {file delete $save}
-	}
 
-	# -- Stash our current window geometry into this repository.
-	#
-	set cfg_geometry [list]
-	lappend cfg_geometry [wm geometry .]
-	lappend cfg_geometry [lindex [.vpane sash coord 0] 1]
-	lappend cfg_geometry [lindex [.vpane.files sash coord 0] 0]
-	if {[catch {set rc_geometry $repo_config(gui.geometry)}]} {
-		set rc_geometry {}
-	}
-	if {$cfg_geometry ne $rc_geometry} {
-		catch {exec git repo-config gui.geometry $cfg_geometry}
+		# -- Stash our current window geometry into this repository.
+		#
+		set cfg_geometry [list]
+		lappend cfg_geometry [wm geometry .]
+		lappend cfg_geometry [lindex [.vpane sash coord 0] 1]
+		lappend cfg_geometry [lindex [.vpane.files sash coord 0] 0]
+		if {[catch {set rc_geometry $repo_config(gui.geometry)}]} {
+			set rc_geometry {}
+		}
+		if {$cfg_geometry ne $rc_geometry} {
+			catch {exec git repo-config gui.geometry $cfg_geometry}
+		}
 	}
 
 	destroy .