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

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>