git-gui: Paper bag fix quitting crash after commit

My earlier introduction of the GITGUI_BCK file (which saves the user's
commit message buffer while they are typing it) broke the Quit function.
If the user makes a commit we delete the GITGUI_BCK file; if they then
immediately quit the application we fail to rename the GITGUI_BCK file
to GITGUI_MSG.  This is because the file does not exist, but our flag
still says it does.  The root cause is we did not unset the flag during
commit.

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

lib/commit.tcl

@@ -381,7 +381,7 @@ A rescan will be automatically started now.
 	$ui_comm edit modified false
 	if {$::GITGUI_BCK_exists} {
 		catch {file delete [gitdir GITGUI_BCK]}
-		set $::GITGUI_BCK_exists 0
+		set ::GITGUI_BCK_exists 0
 	}
 
 	if {[is_enabled singlecommit]} do_quit