gix-status improvements

[Byron]

Based on #1106
This PR is partial and followed by #1155

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diff-correctness → gix-status → gix reset

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Improve gix status to the point where it's suitable for use in reset functinoality.
Leads to a proper worktree reset implementation, eventually leading to a high-level reset similar to how git supports it.

Architecture

The reason this PR deals quite a bit with gix status is that for a safe implementation of reset() we need to be sure that the files we would want to touch don't don't carry modifications or are untracked files. In order to know what would need to be done, we have to diff the current-index with target-index. The set of files to touch can then be used to lookup information provided by git-status, like worktree modifications, index modifications, and untracked files, to know if we can proceed or not. Here is also where the reset-modes would affect the outcome, i.e. what to change and how.

This is a very modular approach which facilitates testing and understanding of what otherwise would be a very complex algorithm. Having a set of changes as output also allows to one day parallelize applying these changes.

This leaves us in a situation where the current checkout() implementation wants to become a fastpath for situations where the reset involves an empty tree as source (i.e. create everything and overwrite local changes).

On the way to reset() it's a valid choice to warm up more with the matter by improving on the current gix status implementation and assure correctness of what's there, which currently doesn't seem to be the case in comparison. Further, implementing gix status similarly to git status should be made possible.

Gix Status

• fun: a way to apply filters in cat-file equivalent, and possibly textconv conversions just like in `git cat-file.
• a way to obtain untracked files to learn if changes can be made.
  • Integrate with the untracked-file extension and provide a way to update it.
• Integrate untracked and ignored files with rename tracking of index-worktree diffs
• status in gix crate with index-worktree
• diff index with index to learn what we would want to do in the worktree, or alternatively,
  diff tree with index (with reverse-diff functionality to simulate diff of index with tree), for better performance as it
  would avoid having to allocate a whole index even though we are only interested in a diff.
  • Must include rename tracking.
• how to make diff results available from status with all transformations applied, to allow user to obtain diffs of any kind?

Next PR: Reset

• reset() that checks if it's allowed to perform a worktree modification is allowed, or if an entry should be skipped. That way we can postpone safety checks like --hard

Postponed

What follows is important for resets, but won't be needed for cargo worktree resets.

• gix status with actual submodule support - needs status in gix (crate) effectively
• gix status with actual conflict support

Research

• Ignored files are considered expandable and can be overwritten on reset
• How to integrate submodules - probably easy to answer once gix status can deal a little better with submodules. Even though in this case a lot of submodule-related information is needed for a complete reset, probably only doable by a higher-level caller which orchestrates it.
• How to deal with various modes like merge and keep? How to control refresh? Maybe partial (only the files we touch), and full, to also update the files we don't touch as part of status? Maybe it's part of status if that is run before.
• Worthwhile to make explicit the difference between git reset and git checkout in terms of HEAD modifications. With the former changing HEADs referent, and the latter changing HEAD itself.
• figure out how this relates to the current checkout() method as technically that's a reset --hard with optional overwrite check. Could it be rolled into one, with pathspec support added?
  • just keep them separate until it's clear that reset() performs just as well, which is unlikely as there is more overhead. But maybe it's not worth to maintain two versions over it. But if so, one should probably rename it.
• for git status: what about rename tracking? It's available for tree-diffs and quite complex on its own. Probably only needs HEAD-vs-index rename tracking. No, also can have worktree rename tracking, even though it's hard to imagine how this can be fast unless it's tightly integrated with untracked-files handling. This screams for a generalization of the tracking code though as the testing and implementation is complex, but should be generalisable.