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OutputPage: Only set <link hreflang=x-default> on wiki/pages with variants

Authored by Krinkle.

Description

OutputPage: Only set <link hreflang=x-default> on wiki/pages with variants

Follows-up 91d8a51825.

Having the default variant advertised seems useful, however for
wikis and/or pages that have no variants or translated versions,
outputting this header doesn't seem useful.

https://support.google.com/webmasters/answer/189077 doesn't
justify sending it unconditionally.

To try out, setting $wgLanguageCode = 'zh'; will result in all
pages having hreflang=x-default and all variants. And setting to
'en' or 'de' will result in none of those link tags.

Change-Id: I21cd072534ae1df960209e657b19c96889ece27c