grep: don't short-circuit -v empty pattern under -x/-w

[MsfPablo]

Fixes #65.

The -v + empty-pattern fast path returned "no lines selected, exit 1" without reading input, on the assumption that an empty pattern matches every line. Under -x the empty pattern matches only an empty line, and under -w only an empty match at a word boundary, so the inversion should still select the remaining lines. Gated the short-circuit on neither flag being set.

$ printf 'abc\ndef\n' | grep -e '' -x -v
abc
def

Diffed against GNU grep across -x/-w combined with -v, -i, -c, -n, -o and -E. All -x cases now agree, including exit codes, and the plain -v short-circuit is untouched (its existing test still passes).

One divergence remains under -w, and it is separate from this change. uu_grep treats an empty line as matching -w with an empty pattern; GNU does not:

$ printf 'abc\n\ndef\n' | grep -e '' -w    # uu: prints the empty line, exit 0
$ printf 'abc\n\ndef\n' | grep -e '' -w    # GNU: no output, exit 1

That reproduces on main with no -v involved, so it is a -w matcher issue rather than a short-circuit one — the consequence is that -w -v still drops empty lines that GNU keeps. I left it alone to keep this change to what the issue describes; happy to open a separate issue for it, or fold a fix in here if you would rather.

Tests cover -x/-w on input without empty lines and -x on input with one. cargo test: 95 passed. clippy -D warnings and cargo fmt --check clean.

[lhecker]

Please revert this comment. IMO the two-line comment is sufficient to explain why the early-return is necessary. The new comment is redundant with the code which already states the logic via && !word_regexp && !line_regexp.

Thanks!