gitk: Disable the head context menu entries for the checked-out branch

Neither the "check out this branch" nor the "remove this branch"
menu item can be used on the currently-checked out branch, so disable
them.

Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -5502,10 +5502,16 @@ proc cherrypick {} {
 
 # context menu for a head
 proc headmenu {x y id head} {
-    global headmenuid headmenuhead headctxmenu
+    global headmenuid headmenuhead headctxmenu mainhead
 
     set headmenuid $id
     set headmenuhead $head
+    set state normal
+    if {$head eq $mainhead} {
+	set state disabled
+    }
+    $headctxmenu entryconfigure 0 -state $state
+    $headctxmenu entryconfigure 1 -state $state
     tk_popup $headctxmenu $x $y
 }
 
@@ -5537,6 +5543,7 @@ proc rmbranch {} {
 
     set head $headmenuhead
     set id $headmenuid
+    # this check shouldn't be needed any more...
     if {$head eq $mainhead} {
 	error_popup "Cannot delete the currently checked-out branch"
 	return