packp: AdvRefs is not encoded correctly when missing Head pointer

[smola]

From https://github.com/git/git/blob/master/Documentation/technical/pack-protocol.txt

If HEAD is a valid ref, HEAD MUST appear as the first advertised
ref.  If HEAD is not a valid ref, HEAD MUST NOT appear in the
advertisement list at all, but other refs may still appear.

So HEAD, if present, should be the first reference. Otherwise, the first reference is encoded as usual.

This is different from our current implementation, which encodes a zero hash instead:

func (s *AdvRefsEncodeSuite) TestRefs(c *C) {
	references := map[string]plumbing.Hash{
		"refs/heads/master":      plumbing.NewHash("a6930aaee06755d1bdcfd943fbf614e4d92bb0c7"),
		"refs/tags/v2.6.12-tree": plumbing.NewHash("1111111111111111111111111111111111111111"),
		"refs/tags/v2.7.13-tree": plumbing.NewHash("3333333333333333333333333333333333333333"),
		"refs/tags/v2.6.13-tree": plumbing.NewHash("2222222222222222222222222222222222222222"),
		"refs/tags/v2.6.11-tree": plumbing.NewHash("5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c"),
	}
	ar := &AdvRefs{
		References: references,
	}

	expected := pktlines(c,
		"0000000000000000000000000000000000000000 capabilities^{}\x00\n",
		"a6930aaee06755d1bdcfd943fbf614e4d92bb0c7 refs/heads/master\n",
		"5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c refs/tags/v2.6.11-tree\n",
		"1111111111111111111111111111111111111111 refs/tags/v2.6.12-tree\n",
		"2222222222222222222222222222222222222222 refs/tags/v2.6.13-tree\n",
		"3333333333333333333333333333333333333333 refs/tags/v2.7.13-tree\n",
		pktline.FlushString,
	)

	testEncode(c, ar, expected)
}

[mcuadros]

@smola still happening?

[mcarmonaa]

PR #552 solves it

[orirawlings]

Can this be closed?