[gix-object] Support empty tags with or without trailing NL

[pierrechevalier83]

When representing an annotated tag with an empty commit message, we used to only support a tag ending with two newlines (one after the tagger line + one after the empty commit message).

This was due to a misconception that annotated tags ending with a single NL shouldn't exist in the wild since there isn't an obvious way to create them with git.
This misconception shows up in the discussion thread for issue 603

It turns out that both encodings of empty annotated tags appear in the wild.

We must be able to parse either and roundtrip for either.

Before PR 604, we used to special case the empty tag msg case and not add a NL. To be able to represent b"", b"\n", b"\n\n"..., we special case any message that is a pure sequence of b'\n' and actually parse the b'\n' into the tag's message.

This allows us to calculate the correct size that matches the number of bytes that git would produce, as well as round-trip to and from the commit encoding.

The existing tests (in particular round_trip for empty.txt and empty_missing_nl.txt) convince me that the logic is sound. Also, the size being 139 bytes for empty_missing_nl.txt and 140 bytes for empty.txt matches the output of cat file | wc -l for each file.

[Byron]

Thanks a lot, that's great!
I definitely had problems getting the parsing right and by now tests are all we have :D.

Regarding the commit I force-pushed, it's only changing the subject, even though it looks like I am the author now.
This is due to the author being strangely formatted:

❯ gix cat @
tree 12ca5ffc6ce6ad1682ecafdd31bb2e223f5ad0b3
parent 6d5f77413e63cad45415d0f674c58d41091bffa1
author Pierre Chevalier pierrec@meta.com <> 1742556630 +0000
committer Sebastian Thiel <sebastian.thiel@icloud.com> 1742613444 +0800
gpgsig -----BEGIN PGP SIGNATURE-----

This already was the case in the original.

❯ gix cat 2435710b4
tree 12ca5ffc6ce6ad1682ecafdd31bb2e223f5ad0b3
parent 6d5f77413e63cad45415d0f674c58d41091bffa1
author Pierre Chevalier redacted@meta.com <> 1742556630 +0000
committer Pierre Chevalier <pierrechevalier-redacted> 1742565757 +0000

Thus I'd leave it to you to change the subject and force-push again so the authorship comes out correctly. And maybe there is a way to fix the author line while at it :).

[pierrechevalier83]

Thus I'd leave it to you to change the subject and force-push again so the authorship comes out correctly. And maybe there is a way to fix the author line while at it :).

Thanks for the fast review. I've fixed the author/committer fields and rebased the change.

[Byron]

Fantastic, thanks! Thanks for the fast turnaround, too!