pathspec: allow to ignore SKIP_WORKTREE entries on index matching
Add a new enum parameter to `add_pathspec_matches_against_index()` and
`find_pathspecs_matching_against_index()`, allowing callers to specify
whether these function should attempt to match SKIP_WORKTREE entries or
not. This will be used in a future patch to make `git add` display a
warning when it is asked to update SKIP_WORKTREE entries.
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> Signed-off-by: Junio C Hamano <gitster@pobox.com>