grep: exit loop immediately, if matching is found in the file for "hg grep -l"
Before this patch, internal function "display()" of "hg grep" is not
efficient for "-l"/"--files-with-matches", because loop is continued,
even after the first matching is found in the specified file.
This patch exits loop immediately, if matching is found for
"--files-with-matches".
In this case, "before is None" is equal to "opts.get('files_with_matches')".