xdiff: make xrecord_t.ptr a uint8_t instead of char

xdiff: make xrecord_t.ptr a uint8_t instead of char

Make xrecord_t.ptr uint8_t because it's referring to bytes in memory.

In order to avoid a refactor avalanche, many uses of this field were
cast to char* or similar.

Places where casting was unnecessary:
xemit.c:156
xmerge.c:124
xmerge.c:127
xmerge.c:164
xmerge.c:169
xmerge.c:172
xmerge.c:178

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