… .gitignore

Switches `require_git(false)` to `require_git(true)` in both the oxlint
and oxfmt walkers, and reverts the `parents(false)` workaround added in
oxc-project#17813 (oxlint only).

**Background:** `require_git(false)` was introduced in oxc-project#17375 so that
`.gitignore` rules would apply even in directories without a `.git` file.
A side-effect is that the `ignore` crate reads `.gitignore` files from
directories *above* the git root, which is non-standard git behaviour.
This caused a concrete bug: when linting inside a nested git repo
(`parent_repo/child_repo/`), the parent repo's `.gitignore` incorrectly
filtered files in the child repo.

**Why require_git(true) is the right fix:** The `ignore` crate's
`matched_ignore` already implements the correct git-boundary logic via a
`saw_git` flag. When iterating parent directories from innermost to
outermost, `saw_git` is set to `true` the moment the innermost `.git`
directory is encountered, preventing gitignore rules from any outer
repository from being applied. Setting `require_git(true)` activates this
standard behaviour.

**Why parents(false) can be reverted (oxlint):** `parents(false)` was a
workaround for the nested-repo bug. With `require_git(true)` the `saw_git`
mechanism handles this correctly, so `parents(true)` can be restored.
This also fixes the regression where running `oxlint src/` would fail to
pick up `.gitignore` rules from the repo root sitting above `src/`.

**Trade-off:** `.gitignore` rules no longer apply in directories that have
no `.git` ancestor. This reverts the behaviour added in oxc-project#17375.

This regressed in oxlint v1.36.0 (fec2863)

This is caused by the walker configuring `ignore` with `.require_git(false)`, which makes `.gitignore` files from parent directories above the current repository apply to the walk. That differs from Git behavior, where `.gitignore` lookup stops at the working tree root.

Here is how we handle it:

- detect whether all walk roots are inside a Git/JJ repository boundary
- enable `require_git(true)` for repository walks so parent `.gitignore` files above the repo boundary are ignored
- keep `require_git(false)` for non-repository walks so local `.gitignore` files still work outside Git repos
- add regression coverage for nested repositories, worktree-style `.git` files, and subdirectory walks inside a repo

Here is how the `ignore` crate checks for git directories:

- when `require_git(true)` is enabled, it records whether each directory has a `.git` or `.jj` entry
- while matching `.gitignore` rules, it applies parent gitignore matchers only until it has seen that repository boundary
- `.git` files are handled as boundaries too, which covers Git worktrees

Fixes #17510.
Fixes #17805.
Fixes #19902.
Closes #21727.
Closes #17513.
Closes #17813.
Closes #20951.
Closes #20952.