gitk: Prevent the text pane from becoming editable

When setting the "Patch/Tree" radio buttons to "Tree" and
clicking on a file to display it, the text pane would
accidentally become editable (because of the early return
in getblobline).

Signed-off-by: Stefan Haller <stefan@haller-berlin.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -7327,6 +7327,7 @@ proc getblobline {bf id} {
 			    [lindex [split $commentend .] 0]}]
 	    mark_ctext_line $lnum
 	}
+	$ctext config -state disabled
 	return 0
     }
     $ctext config -state disabled