Make it possible for a single cmd to work both with and without a repo

When cgit_cmd.want_repo was 0, we used to assume that the cmd would never
be invoked for a repo. But soon this will become untrue (the 'about' cmd
is rapidly approching), so from now on we will initialize any requested
repo even if want_repo==0 (and return an error if want_repo==1 but no repo
is specified).

Signed-off-by: Lars Hjemli <hjemli@gmail.com>