pathspec.h: move pathspec_needs_expanded_index() from reset.c to here
Method pathspec_needs_expanded_index() in reset.c from
4d1cfc1351
(reset: make --mixed sparse-aware, 2021-11-29) is reusable when we
need to verify if the index needs to be expanded when the command
is utilizing a pathspec rather than a literal path.
Move it to pathspec.h for reusability.
Add a few items to the function so it can better serve its purpose as
a standalone public function:
* Add a check in front so if the index is not sparse, return early since
no expansion is needed.
* It now takes an arbitrary 'struct index_state' pointer instead of
using `the_index` and `active_cache`.
* Add documentation to the function.
Helped-by: Victoria Dye <vdye@github.com>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>