pickaxe: pass diff_options to contains and has_changes

pickaxe: pass diff_options to contains and has_changes

Remove the unused parameter needle from contains() and has_changes().

Also replace the parameter len with a pointer to the diff_options.  We
can use its member pickaxe to check if the needle is an empty string
and use the kwsmatch structure to find out the length of the match
instead.

This change is done as a preparation to unify the signatures of
has_changes() and diff_grep(), which will be used in the patch after
the next one to factor out common code.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>