Apply path_filter while writing the scoped filelist instead of only after
grep, so large indexed projects do not scan files whose hits the post-grep
filter would discard anyway. The prefilter runs the SAME compiled regex via
the same cbm_regexec call against the same root-relative path (Windows
separators normalized first) as the post-grep filter in collect_grep_matches,
which is kept as belt-and-suspenders — results-preserving by construction.
Guard the edge the prefilter introduces: a path_filter matching zero indexed
files now short-circuits with the normal empty result instead of invoking
xargs on an empty filelist (GNU execs grep once with no operands, BSD skips).
Tests: positive guard (filter matching the hit's file still returns it,
out-of-filter file excluded) and zero-match guard (clean empty response,
grep subprocess skipped). Both green pre-change too — perf-only change.
Distilled from #756 rebased onto the post-#751 filelist writer.
Co-authored-by: yutianyu111602-glitch <yutianyu111602@gmail.com>
Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>
