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, 1 insertions, 0 deletions

diff --git a/plumbing/protocol/packp/updreq.go b/plumbing/protocol/packp/updreq.go
index 0624930..b246613 100644
--- a/plumbing/protocol/packp/updreq.go
+++ b/plumbing/protocol/packp/updreq.go
@@ -42,6 +42,7 @@ func NewReferenceUpdateRequest() *ReferenceUpdateRequest {
 //   - report-status
 //   - ofs-delta
 //   - ref-delta
+//   - delete-refs
 // It leaves up to the user to add the following capabilities later:
 //   - atomic
 //   - ofs-delta