[D7] Class active-trail not added to li element when linking to front page

From parent D8 issue:
#1578832: [already commited but followup for CR?] <front> menu links are missing active trail classes

There is a bug with the "active-trail" class not showing up on <li> for menu items with Drupal 7.14. The problem is that "active-trail" is missing for a menu link to the front page (path "<front>"), all other links have "active-trail" just fine.

The situation is also slightly different for the horizontal primary link tabs that can be switched on from theme settings Toggle display - Main menu, than they are for the vertical navigation in the Main menu block. In the horizontal tabs the front page <li> has "active" but not "active-trail". So "active" could be used to do get around this issue for horizontal menus. But unfortunately if you want the vertical menu, there is no "active" nor "active-trail".

I verified that both "active" and "active-trail" are missing with a fresh install without any added modules on Drupal 7.4 to 7.14. The behaviour is identical with Bartik, Garland and Stark themes, as well as Zen.

Steps to reproduce:

1. Create a fresh install of Drupal 7.
2. On the front page go to Structure -> Blocks.
3. Move "Main menu" block to "First sidebar"
4. Create a new "Basic page" with test content and click "Provide a menu link" -> "Parent item" [Main menu]
5. Use e.g. Firebug to verify that the newly created test page link on the sidebar has "active-trail" on <li> but "Home" does not.

Attached is a patch that fixes the bug by adding special handling for the front page when adding the active-trail class. A better solution would be to get the 'in_active_trail' variable correctly set for links to <front>, but unfortunately I could not find out how to do that.