gitweb: group remote heads by remote

gitweb: group remote heads by remote

In remote and summary view, display a block for each remote, with the
fetch and push URL(s) as well as the list of the remote heads.

In summary view, if the number of remotes is higher than a prescribed
limit, only display the first <limit> remotes and their fetch and push
urls, without any heads information and without grouping.

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