Item11267: FoswikirefsPlugin for new repo layout

Implements new macros
 - %GITREF% links to an account:repo:commithash on github
 - %REV2REF% Outputs multiple GITREF macros for a svn revision.

The complete xref of svn to git commits is generated as part of this
extension.

git-svn-id: http://svn.foswiki.org/trunk@17897 0b4bb1d4-4e5a-0410-9cc4-b2b747904278