Before this patch, when ACL is involved, narrowspecs are send as bundle2
parameter for narrow:spec bundle2 part. The limitation of bundle2 parts are they
cannot send data larger than 255 bytes. Includes and excludes in narrow are not
limited by size and they can grow over 255 bytes.
This patch introduces a new mandatory bundle2 part and send narrowspecs as data
of that. The new bundle2 part is introduced to keep things cleaner and easy to
distinguish related to backward compatibility.
The part is mandatory because without server's narrowspec, the local ACL narrow
repo won't work.
This patch makes clients compatible with servers which have older versions.
However I left a comment that we should drop the other bundle2 part soon as
that's broken and people should not rely on that.
I named the new bundle2 part 'Narrowspec' because:
1) Capital 'N' to make it mandatory
2) 'Narrow:spec' cannot be used because bundle2 enforces that there should not
be two different parts which resolve to same name when lowercased.