gitweb: separate heads and remotes lists

gitweb: separate heads and remotes lists

We specialize the 'heads' action to only display local branches, and
introduce a 'remotes' action to display the remote branches (only
available when the remotes_head feature is enabled).

Mirroring this, we also split the heads list in summary view into
local and remote lists, each linking to the appropriate action.

The git_get_heads_list now defaults to 'heads' only, regardless of
whether the remote heads feature is active or not.

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