git-gui: Correct window title for hook failure dialogs

During i18n translation work this message was partially broken
by using "append" instead of "strcat" to join the two different
parts of the message together.

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

lib/error.tcl

@@ -99,6 +99,6 @@ proc hook_failed_popup {hook msg} {
 
 	bind $w <Visibility> "grab $w; focus $w"
 	bind $w <Key-Return> "destroy $w"
-	wm title $w [append "[appname] ([reponame]): " [mc "error"]]
+	wm title $w [strcat "[appname] ([reponame]): " [mc "error"]]
 	tkwait window $w
 }