fix(gix-index): correctly decode untracked extension

[Aaron Moat (AaronMoat)]

• Fix ctime/mtime flip in stat decoding
• Fix incomplete decoding to mirror git, with associated ported git test

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Split out from #2503 as requested. I thought including the untracked cache parsing end-to-end was small enough and more logical than just the ctime/mtime flip -- as that's the only spot it's used anyway. Hope that's okay.

The tests added are limited; most of git's tests are focused on its interactions with git status. The tests added are attempting to replicate these.

As for the ctime/mtime Sebastian Thiel (@Byron):

Can we land this separately? Ideally with a test if that's possible. I remember that some strange swapping was going on when decoding, but I always thought it was necessary. Maybe I got it wrong and no test caught it. Let's take a separate look.

I'm not really sure what we should test: I don't see an obvious git test to port. As for correctness, it seems that the data is just read into this struct, which is ctime then mtime. The existing code was reading ctime then mtime variables correctly, then assigning the reverse into the rust struct.

Let me know if there's any test suggestions, happy to extend them. I don't know the git source code very well so may very well be missing some good tests there.

[Sebastian Thiel (Byron)]

Tasks

• review - double-check
• split gix-testtools feature out into own commit
• add XDG_CONFIG_HOME override

[Copilot]

Pull request overview

This PR fixes gix-index’s decoding of the index UNTR (untracked cache) extension to match Git’s on-disk layout, and corrects a ctime/mtime swap in stat decoding. It also adds fixtures + snapshot-style tests to validate end-to-end decoding of the untracked cache directory graph.

Changes:

• Fix decode::stat() to assign decoded ctime/mtime to the correct entry::Stat fields.
• Adjust UNTR decoding to read both stats, then dir_flags, then both hashes (matching Git), and add public accessors + Debug derives for snapshotting.
• Add new archived fixtures + tests for empty/populated/nested untracked cache decoding; update testtools to support “must use archive” fixtures and isolate XDG_CONFIG_HOME.

Reviewed changes

Copilot reviewed 11 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/tools/src/tests.rs	Adds a test to ensure configure_command() overrides XDG_CONFIG_HOME.
tests/tools/src/lib.rs	Adds “needs archive” fixture helper; ensures fixture commands run with a private XDG_CONFIG_HOME; threads needs_archive through archive extraction.
gix-index/tests/index/main.rs	Adds a fixture helper that forces using archived fixtures even when archives are otherwise ignored.
gix-index/tests/index/file/read.rs	Adds snapshot tests (via insta) for decoded UNTR extension shapes (empty/populated/nested) using archived fixtures.
gix-index/tests/fixtures/make_index/untracked_cache_empty.sh	New fixture generator for an index with an empty UNTR cache.
gix-index/tests/fixtures/make_index/untracked_cache_populated.sh	New fixture generator producing a populated UNTR cache via git status.
gix-index/tests/fixtures/make_index/untracked_cache_nested.sh	New fixture generator covering nested directories and per-dir ignore OIDs in UNTR.
gix-index/src/extension/untracked_cache.rs	Implements Git-aligned UNTR decode layout; adds Debug derives and accessor methods for snapshot-friendly inspection.
gix-index/src/extension/mod.rs	Derives Debug for UntrackedCache.
gix-index/src/decode/mod.rs	Fixes ctime/mtime field assignment in stat decoding.
gix-index/Cargo.toml	Adds insta as a dev-dependency for new snapshot tests.
Cargo.lock	Locks new dev-dependency additions (e.g., insta/transitives).

[Copilot]

Pull request overview

Copilot reviewed 14 out of 18 changed files in this pull request and generated 2 comments.

[Sebastian Thiel (Byron)]

Just as an FYI for Eliah Kagan (@EliahKagan), This pr also adds an extension to gix-testtools which allows to force the usage of an archive. This is useful if some data created by the underlying shell script is inherently unstable and not under our control.
Overall, I think that's a better system than the one I used previously, which is to copy the artifacts into the fixture directory and track them without any indication of how they were created or where they are from.

[Sebastian Thiel (Byron)]

Oh, and now that I see it: Thanks a lot for making this happen Aaron Moat (@AaronMoat), it's much appreciated!
I didn't want to seem ungrateful. I just forgot to write this message yesterday after a long day spent with this PR.

It's interesting that in the age of agentic, you don't necessarily need less time to get into something, because you're still the puny human you were before, but instead you get more done in that time as you can just instruct the agent to do it real quick. While I look, play, investigate, watch and learn, there's always an agent doing work for me. And that then has to be reviewed again, so there's more work and more work being created on the fly, which just didn't happen before 😁.
But also that work represents value and overall the project project moves much faster than it could otherwise, as any barriers (like "nah, this seems difficult and there are too many unknowns to tackle this") are now gone.