A remote can have a multitude of refspecs. Up to now our git_remote's
have supported a single one for each fetch and push out of simplicity
to get something working.
Let the remotes and internal code know about multiple remotes and get
the tests passing with them.
Instead of setting a refspec, the external users can clear all and add
refspecs. This should be enough for most uses, though we're still
missing a querying function.
* gen_pktline() in smart_protocol.c was skipping refspecs that deleted
refs that were not advertised by the server. The new behavior is to
send a delete command with an old-id of zero, which matches the behavior
of the official git client.
* Update test_network_push__delete() in reaction to above fix.
* Obviate messy logic that handles missing push_spec rrefs by canonicalizing
push_spec. After calculate_work(), loid, roid, and rref, are filled in with
exactly what is sent to the server
