ls-files: filter pathspec before lstat

ls-files: filter pathspec before lstat

In --deleted and --modified modes, show_files() calls lstat() for each
index entry before show_ce() applies the pathspec. prune_index() avoids
most of these calls for pathspecs with a common directory prefix, but
not for a top-level name or leading wildcard.

Match before lstat() to avoid accessing the worktree for entries that
cannot be shown. Treat this as a prefilter: do not update ps_matched,
and retain the match in show_ce() so --error-unmatch is satisfied only
by entries that the selected modes actually show.

Prefilter only a single pathspec item, bounding the added work for each
index entry. Applying match_pathspec() to multiple arguments can cost
more than the lstat() calls it avoids. In a synthetic repository with
10,000 clean files, passing every path to ls-files --modified increased
runtime from 112.5 ms to 494.1 ms when the prefilter was unconditional.

With $parent and $this exported as paths to binaries built from the
parent and this commit, on a repository with 881,290 index entries:

    hyperfine --warmup 0 --runs 3 \
        --command-name parent \
        '$parent -c core.fsmonitor=false ls-files --deleted -- README.md >/dev/null' \
        --command-name this-commit \
        '$this -c core.fsmonitor=false ls-files --deleted -- README.md >/dev/null'

reported means of 65.790 seconds for the parent and 4.987 seconds for
this commit.

Link: https://lore.kernel.org/r/xmqqfr2tnfk0.fsf@gitster.g
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>