xdiff: delete struct diffdata_t

xdiff: delete struct diffdata_t

Every field in this struct is an alias for a certain field in xdfile_t.

diffdata_t.nrec   -> xdfile_t.nreff
diffdata_t.ha     -> xdfile_t.ha
diffdata_t.rindex -> xdfile_t.rindex
diffdata_t.rchg   -> xdfile_t.rchg

I think this struct existed before xdfile_t, and was kept for backward
compatibility reasons. I think xdiffi should have been refactored to
use the new (xdfile_t) struct, but was easier to alias it instead.

The local variables rchg* and rindex* don't shorten the lines by much,
nor do they really need to be there to make the code more readable.
Delete them.

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