git-gui: Don't quit when we destroy a child widget

Its wrong to exit the application if we destroy a random widget
contained withing something else; especially if its some small
trivial thing that has no impact on the overall operation.

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

git-gui.sh

@@ -1600,7 +1600,7 @@ unset browser doc_path doc_url
 
 # -- Standard bindings
 #
-bind .   <Destroy> do_quit
+bind .   <Destroy> {if {{%W} eq {.}} do_quit}
 bind all <$M1B-Key-q> do_quit
 bind all <$M1B-Key-Q> do_quit
 bind all <$M1B-Key-w> {destroy [winfo toplevel %W]}