gitweb: disambiguate heads and tags withs the same name

gitweb: disambiguate heads and tags withs the same name

Avoid wrong disambiguation that would link logs/trees of tags and
heads which share the same name to the same page, leading to
a disambiguation that would prefer the tag, thus making it impossible
to access the corresponding head log and tree without hacking the url
by hand.

It does it by using full refname (with 'refs/heads/' or 'refs/tags/'
prefix) instead of shortened one in the URLs in 'heads' and 'tags'
tables.  This makes URLs (and refs) provided by gitweb unambiguous.

Signed-off-by: Guillaume Seguin <guillaume@segu.in>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>