gitk: Ensure that "Reset branch" menu entry is enabled

Consider this sequence of events:

1. Detach HEAD and fire up gitk
2. Call the context menu on some commit. Notice that the last menu entry
   says "Detached HEAD: can't reset" and it is disabled.
3. Now checkout some regular branch (e.g. 'master') using the context menu.
4. Call the context menu again on some commit.

Previously, at this point the last menu entry said "Reset master branch
to here", but it was still disabled. With this fix it is now enabled again.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -7979,7 +7979,7 @@ proc rowmenu {x y id} {
     if {$id ne $nullid && $id ne $nullid2} {
 	set menu $rowctxmenu
 	if {$mainhead ne {}} {
-	    $menu entryconfigure 7 -label [mc "Reset %s branch to here" $mainhead]
+	    $menu entryconfigure 7 -label [mc "Reset %s branch to here" $mainhead] -state normal
 	} else {
 	    $menu entryconfigure 7 -label [mc "Detached head: can't reset" $mainhead] -state disabled
 	}