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, 0 insertions, 5 deletions

diff --git a/plumbing/transport/server/receive_pack_test.go b/plumbing/transport/server/receive_pack_test.go
index 73ba60b..54c2fba 100644
--- a/plumbing/transport/server/receive_pack_test.go
+++ b/plumbing/transport/server/receive_pack_test.go
@@ -27,11 +27,6 @@ func (s *ReceivePackSuite) TearDownTest(c *C) {
 	s.Suite.TearDownSuite(c)
 }
 
-// TODO
-func (s *ReceivePackSuite) TestSendPackAddDeleteReference(c *C) {
-	c.Skip("delete reference not supported yet")
-}
-
 // Overwritten, server returns error earlier.
 func (s *ReceivePackSuite) TestAdvertisedReferencesNotExists(c *C) {
 	r, err := s.Client.NewReceivePackSession(s.NonExistentEndpoint, s.EmptyAuth)