pathspec.c: rename newly public functions for clarity

Perform the following function renames to make it explicit that these
pathspec handling functions are for matching against the index, rather
than against a tree or the working directory.

- fill_pathspec_matches() -> add_pathspec_matches_against_index()
- find_used_pathspec() -> find_pathspecs_matching_against_index()

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>