gitk: disable colours when calling git log

If the user specifies 'diff.color = 1' in their configuration file,
then gitk will not start.  Disable colours when calling git log.

Signed-off-by: Sam Vilain <sam.vilain@catalyst.net.nz>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

gitk

@@ -92,7 +92,7 @@ proc start_rev_list {view} {
 	set order "--date-order"
     }
     if {[catch {
-	set fd [open [concat | git log -z --pretty=raw $order --parents \
+	set fd [open [concat | git log --no-color -z --pretty=raw $order --parents \
 			 --boundary $viewargs($view) "--" $viewfiles($view)] r]
     } err]} {
 	error_popup "Error executing git rev-list: $err"