gitweb: Include links to feeds in HTML header only for '200 OK' response

To do that, generating "<link />"s to feeds were refactored into
print_feed_meta() subroutine, to keep nesting (indent) level in
git_header_html() low.  This has also the advantage of making code
more clear.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: John 'Warthog9' Hawley <redacted>
Signed-off-by: Junio C Hamano <redacted>