git-gui: Fix the search bar destruction handler.

Since delete_this is an ordinary function, it
should not be passed to cb; otherwise it produces
errors when blame windows are closed. Unfortunately,
it is not noticeable when blame is shown in the
master window, so I missed this bug.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Author: angavrilov

lib/search.tcl

@@ -35,7 +35,7 @@ constructor new {i_w i_text args} {
 
 	trace add variable searchstring write [cb _incrsearch_cb]
 
-	bind $w <Destroy> [cb delete_this]
+	bind $w <Destroy> [list delete_this $this]
 	return $this
 }