xdiff: remove dependence on xdlclassifier from xdl_cleanup_records()

xdiff: remove dependence on xdlclassifier from xdl_cleanup_records()

Disentangle xdl_cleanup_records() from the classifier so that it can be
moved from xprepare.c into xdiffi.c.

The classic diff is the only algorithm that needs to count the number
of times each line occurs in each file. Make xdl_cleanup_records()
count the number of lines instead of the classifier so it won't slow
down patience or histogram.

Signed-off-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>