stash: avoid sparse-index expansion for in-cone paths

`git stash push -- <pathspec>` expands a sparse index before checking whether the pathspec matches any tracked paths. This is unnecessary when the pathspec is wholly inside the sparse-checkout cone and makes a path-limited stash proportional to the size of the full index. Use `pathspec_needs_expanded_index()` to expand only when a pathspec can match part of a sparse-directory entry, as `git rm` and `git reset` already do. Keep the full-index behavior for pathspecs that need it. Add compatibility coverage for literal, prefixed, wildcard, file, multiple, staged, and missing pathspecs. Add the corresponding path-limited stash case to p2000. On a cone-mode repository with 349,525 tracked paths and 49 sparse index entries, the best of three runs changed from 18.87s to 0.06s. Trace2 reported four index expansions before this change and none after it. Signed-off-by: Ted Nyman <tnyman@openai.com> Reviewed-by: Taylor Blau <ttaylorr@openai.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>