gitweb: introduce remote_heads feature

With this feature enabled, remote heads are retrieved (and displayed)
when getting (and displaying) the heads list. Typical usage would be for
local repository browsing, e.g. by using git-instaweb (or even a more
permanent gitweb setup), to check the repository status and the relation
between tracking branches and the originating remotes.

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