gitweb: nagivation menu for tags, heads and remotes

tags, heads and remotes are all views that inspect a (particular class
of) refs, so allow the user to easily switch between them by adding
the appropriate navigation submenu to each view.

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