Allow relative inclusions in Main namespace.

In previous versions of MediaWiki, relative inclusions ({{../name}})
were not handled properly in the Main namespace - it tried to include
Template:Parent/name instead of just Parent/name article. In other
namespaces they worked, though (all in case of enabled subpages). The
patch fixes this inconsistence and allows to use relative inclusions
in Main.

Change-Id: Ie04f23c180e501631c629a39b997796a1725fb67