grep: -L should show empty files

grep: -L should show empty files

The -L (--files-without-match) option is supposed to show paths that
produced no matches.  When running the internal grep on work tree files,
however, we had an optimization to just return on zero-sized files,
without doing anything.

This optimization doesn't matter too much in practice (a tracked empty
file must be rare, or there is something wrong with your project); to
produce results consistent with GNU grep, we should stop the optimization
and show empty files as not having the given pattern.

Signed-off-by: Junio C Hamano <gitster@pobox.com>