gitweb: git_get_heads_list accepts an optional list of refs

git_get_heads_list(limit, class1, class2, ...) can now be used to retrieve
refs/class1, refs/class2 etc. Defaults to ('heads', 'remotes') or ('heads')
depending on whether the 'remote_heads' feature is enabled or not.

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