gitk: Enable mouse horizontal scrolling in diff pane

Currently it's required to hold Shift and scroll up and down to move
horizontally. Listen to Button-6 and Button-7 events too to make
horizontal scrolling handier with touchpads and some mice.

Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -2516,6 +2516,13 @@ proc makewindow {} {
     } else {
 	bindall <ButtonRelease-4> "allcanvs yview scroll -5 units"
 	bindall <ButtonRelease-5> "allcanvs yview scroll 5 units"
+	bind $ctext <Button> {
+	    if {"%b" eq 6} {
+		$ctext xview scroll -5 units
+	    } elseif {"%b" eq 7} {
+		$ctext xview scroll 5 units
+	    }
+	}
         if {[tk windowingsystem] eq "aqua"} {
             bindall <MouseWheel> {
                 set delta [expr {- (%D)}]