gitweb: remotes view for a single remote

When 'remotes' view is passed the 'hash' parameter, interpret it as the
name of a remote and limit the view the the heads of that remote.

In single-remote view we let the user switch easily to the default
remotes view by specifying an -action_extra for the page header and by
enabling the 'remotes' link in the reference navigation submenu.

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