Currently, when matched with multiple lines by DFA superset, return to
KWset.  However, it won't be wrong probably, because if matches with
multiple lines by DFA superset, also matches with single line there with
high probability.  Further more, if return to KWset after matched with
multiple line by DFA superset, dfafast won't work effectively.

This patch changes to retry DFA superset immediately after matched with
multiple lines by it.

I confirmed the patch by following tests.

  $ yes abcdabc | head -50000000 >k
  $ env LC_ALL=C time -p src/grep '\(ab\)cd\1d' k

  before:
    real 3.48       user 3.41       sys 0.06

  after:
    real 2.14       user 2.07       sys 0.06

Norihiro