Dulwich sending unborn to a server not supporting it

[Lordshinjo]

Using dulwich 1.1.0 with Gerrit 3.12.2, dulwich is sending a unborn packet while cloning a repository, much to the confusion of the server.

Here's a log of what happens:

Details

2026-02-27 21:20:37,463:urllib3.connectionpool:https://[snip]:443 "GET /a/[snip]/info/refs?service=git-upload-pack HTTP/1.1" 200 113
2026-02-27 21:20:37,464:dulwich.protocol:git< b'version 2'
2026-02-27 21:20:37,465:dulwich.protocol:git< b'ls-refs'
2026-02-27 21:20:37,465:dulwich.protocol:git< b'fetch=shallow'
2026-02-27 21:20:37,465:dulwich.protocol:git< b'server-option'
2026-02-27 21:20:37,465:dulwich.protocol:git< b'agent=JGit/v7.4.0.202507080956-m1-15-g010858e24'
2026-02-27 21:20:37,466:dulwich.protocol:git< 0000
2026-02-27 21:20:37,741:urllib3.connectionpool:https://[snip]:443 "POST /a/[snip]/git-upload-pack HTTP/1.1" 200 25
2026-02-27 21:20:37,741:dulwich.protocol:git< b'ERR unexpected unborn'
2026-02-27 21:20:37,741:root:unknown part in pkt-ref: b'unborn'
Traceback (most recent call last):
  File "[snip].py", line 450, in init
    repo = git.clone(
           ^^^^^^^^^^
  File "/[snip]/lib/python3.12/site-packages/dulwich/porcelain/__init__.py", line 1507, in clone
    repo = client.clone(
           ^^^^^^^^^^^^^
  File "/[snip]/lib/python3.12/site-packages/dulwich/client.py", line 1497, in clone
    result = self.fetch(
             ^^^^^^^^^^^
  File "/[snip]/lib/python3.12/site-packages/dulwich/client.py", line 1631, in fetch
    result = self.fetch_pack(
             ^^^^^^^^^^^^^^^^
  File "/[snip]/lib/python3.12/site-packages/dulwich/client.py", line 4660, in fetch_pack
    refs, server_capabilities, url, symrefs, _peeled = self._discover_references(
                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/[snip]/lib/python3.12/site-packages/dulwich/client.py", line 4433, in _discover_references
    (refs, symrefs, peeled) = read_pkt_refs_v2(proto.read_pkt_seq())
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/[snip]/lib/python3.12/site-packages/dulwich/client.py", line 500, in read_pkt_refs_v2
    for pkt in pkt_seq:
               ^^^^^^^
  File "/[snip]/lib/python3.12/site-packages/dulwich/protocol.py", line 526, in read_pkt_seq
    pkt = self.read_pkt_line()
          ^^^^^^^^^^^^^^^^^^^^
  File "/[snip]/lib/python3.12/site-packages/dulwich/protocol.py", line 458, in read_pkt_line
    raise HangupException
dulwich.errors.HangupException: The remote server unexpectedly closed the connection.

The unborn packet sending was added in #2074, where unborn support is determined in the following code:

dulwich/dulwich/client.py

Lines 457 to 460 in f8f714b

	# Check if server supports unborn refs
	supports_unborn = any(
	b"ls-refs=unborn" in cap or b"ls-refs" in cap for cap in server_capabilities
	)

Here the server only sent ls-refs and not ls-refs=unborn.
Would it make more sense to either avoid sending unborn if the server does not advertise it, or possibly discover it some other way?