gitweb: use href() when generating URLs in OPML

Since the OPML project list view was hand-coding the RSS and HTML URLs,
it didn't respect global options such as use_pathinfo. Make it use
href() to ensure consistency with the rest of the gitweb setup.

Signed-off-by: Giuseppe Bilotta <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>