improve delete support on push

* server: implement delete-refs and announce it.
* remote: check if server announced delete-refs before trying
  to delete and fail fast if it does not.

Note that the client does not need no send 'delete-refs' back
to the server to be able to delete references:

```
delete-refs
-----------

If the server sends back the 'delete-refs' capability, it means that
it is capable of accepting a zero-id value as the target
value of a reference update.  It is not sent back by the client, it
simply informs the client that it can be sent zero-id values
to delete references.
```

So our server implementation does not check if the client sent
delete-refs back, it just accepts deletes if it receives them.

1 files changed, 4 insertions, 0 deletions

diff --git a/plumbing/transport/test/receive_pack.go b/plumbing/transport/test/receive_pack.go
index bb1c58a..15172c8 100644
--- a/plumbing/transport/test/receive_pack.go
+++ b/plumbing/transport/test/receive_pack.go
@@ -308,6 +308,10 @@ func (s *ReceivePackSuite) testSendPackDeleteReference(c *C) {
 		req.Capabilities.Set(capability.ReportStatus)
 	}
 
+	if !ar.Capabilities.Supports(capability.DeleteRefs) {
+		c.Fatal("capability delete-refs not supported")
+	}
+
 	c.Assert(r.Close(), IsNil)
 
 	s.receivePack(c, s.Endpoint, req, nil, false)