discard changes: hunks part 2

[Byron]

Add V3 facilities for discarding changes in the worktree or index, this time it's about hunks specifically.

Follow-up of #7752.

Tasks

• reset changed hunks in a file
  • assure test for executable bits
  • test for a dropped hunk
  • should the index be updated after all? - No
  • figure out what's wrong with the hunk application - it already does the wrong thing 'in the middle', maybe it never worked 100% correctly, or maybe there are assumptions an additive approach?
• use latest version of gix - nothing essential

Notes for the reviewer

• I find it difficult to imagine how meaningful discarding parts of hunks in whole-file deletions and additions can be so I chose to disallow it. The UI can still offer discarding the single whole hunk that a deletion or addition of a file would come with, but internally it, as always, would discard the file instead. Discarding of lines or selections would have to be disallowed on the UI level though, as this is currently only available for modified (or renamed + modified) files.

For the next PRs

• reset individual lines (but as free selections of sub-hunks)
  • How does this work with context lines? In theory, they will mess everything up so it's probably something to avoid. After all, they will combine multiple hunks into one and the UI really would have to provide hunks without them.
• See what happens if one tries to commit these special cases (i.e. the 'pretend there is no index' changes)
  • implement remaining ignored cases
• gitoxide informs about diff-filter changes and this information is passed to the frontend
  • differentiate binary-to-text , otherwise it applies worktree conversions (which is fine). We ignore external diff programs anyway.

Shortcomings

• Index-handling is low-level and I think there needs to be something like a tree-editor, but for indices. Maybe it's a mix of using the pretty-neat tree-editor, and a way to transfer index information from one index to another, similar to 'apply', to avoid loosing information.

Out of Scope

• snapshot integration - should be left to @krlvi to warm up with the new code.

Research

• V2 implementation
  • spread across unapply_lines, unapply_ownership and reset_files

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