Add HunkHeader and DiffLineType to ConsumeHunk::consume_hunk

[cruessler]

This PR changes ConsumeHunk::consume_hunk to accept typed data instead of strings.

This PR was initially motivated by gitui-org/gitui#2685.

Initial description (no longer accurate as of 2025-08-24)

This PR is an attempt at extracting a part of UnifiedDiff, more specifically a part that exposes an API for dealing with structured data as opposed to string-based data. It introduces ConsumeTypedHunk which is very similar to ConsumeHunk, except that is uses HunkHeader and DiffLineType to encode diff-related information in proper types.

There are still a few open issues/questions:

• ConsumeTypedHunk is not NewlineSeparator-aware. Is that a good thing or a bad thing? I’m leaning more towards the former.
• The PR contains a few TODO: comments.

What do you think about the PR’s overall design? If you think this works, I’d start addressing the remaining issues.

[cruessler]

I’m going to mark this PR as ready for review in order to indicate that I’m now done with the initial version and would welcome a first round of feedback!

There’s a few genuine TODO: comments in places where I had questions myself. Feel free to mark this PR as draft in case you don’t think it’s ready yet!

[Byron]

Some notes from our session:

• UnifiedDiffSink -> UnifiedDiff - make it a breaking change
• .collect() - reuse the buffer for lines instead.

[cruessler]

I’ve now updated the PR.

• The API is a bit more verbose as the responsibility for handling newlines was moved to the consumer. This had become necessary as HunkHeader::to_string() doesn’t include any newline at all. For consistency, newlines for the actual lines in a diff have to be handled by the consumer as well.
• I did not manage to move buffer2 to UnifiedDiff as I ran into lifetime and ownership issues, but I might have missed something. Please check carefully! :-)