gitk: Add support for -G'regex' pickaxe variant

git log -G'regex' is a very useful alternative to the classic
pickaxe.  Minimal patch to make it usable from gitk.

[zj: reword message]
[paulus@samba.org: reword droplist item]
Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -2253,7 +2253,8 @@ proc makewindow {} {
     set gm [makedroplist .tf.lbar.gdttype gdttype \
 		[mc "containing:"] \
 		[mc "touching paths:"] \
-		[mc "adding/removing string:"]]
+		[mc "adding/removing string:"] \
+		[mc "changing lines matching:"]]
     trace add variable gdttype write gdttype_change
     pack .tf.lbar.gdttype -side left -fill y
 
@@ -4658,6 +4659,8 @@ proc do_file_hl {serial} {
 	set gdtargs [concat -- $relative_paths]
     } elseif {$gdttype eq [mc "adding/removing string:"]} {
 	set gdtargs [list "-S$highlight_files"]
+    } elseif {$gdttype eq [mc "changing lines matching:"]} {
+	set gdtargs [list "-G$highlight_files"]
     } else {
 	# must be "containing:", i.e. we're searching commit info
 	return