gitweb: rss channel date

gitweb: rss channel date

The RSS 2.0 specifications defines not one but _two_ dates for its
channel element! Woohoo! Luckily, it seems that consensus seems to be
that if both are present they should be equal, except for some very
obscure and discouraged cases. Since lastBuildDate would make more sense
for us and pubDate seems to be the most commonly used, we defined both
and make them equal.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>