Fixes#12216
Fixes a panic in `AbsolutePathBuf::parent()` when the process hits file
descriptor exhaustion (`EMFILE` / "Too many open files").
### Root cause
`AbsolutePathBuf::parent()` was re-validating the parent path via
`from_absolute_path(...).expect(...)`.
`from_absolute_path()` calls `path_absolutize::absolutize()`, which can
depend on `std::env::current_dir()`. Under `EMFILE`, that can fail,
causing `parent()` to panic even though the parent of an absolute path
is already known.
### Change
- Stop re-absolutizing the result of `self.0.parent()`
- Construct `AbsolutePathBuf` directly from the known parent path
- Keep an invariant check with `debug_assert!(p.is_absolute())`
### Why this is safe
`self` is already an `AbsolutePathBuf`, so `self.0` is
absolute/normalized. The parent of an absolute path is expected to be
absolute, so re-running fallible normalization here is unnecessary and
can introduce unrelated panics.
a4076ab4b1