(bug 7958) Special:Cite of older version of an article should use old version id.

(bug 7958) Special:Cite of older version of an article should use old version id.
Applying Brion's patch, with one-line tweak:
 - uses the existing Skin/$wgOut-based revision ID record, rather than oldid value from WebRequest.
 - additionally sets it to the current revision for parser cache hits (where
   it was not previously needed, since it was only used to feed to parser objects
   to fill the {{REVISIONID}} variable).
 - Explicit declaration of the existing $mRevisionId data member in Skin.
 - "Permanent link" should now work too when paging through historical versions (previously it
   would be greyed out when paging backwards or forwards through old revisions).