Use git_get_referencing to get HTML code for markers showing which
refs (tags and heads) point to current commit.  It would be much
easier to change format of markers in one or two places than thorough
the gitweb.cgi file.

Added comment about read_info_ref subroutine: for $type == "" (empty
argument) it saves only last path part of ref name e.g. from
'refs/heads/jn/gitweb' it would leave only 'gitweb'.

Some reordering.  Added $ref in one place.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>