diff-delta: use realloc instead of xrealloc

Commit 83572c1a914d3f7a8dd66d954c11bbc665b7b923 changed many
realloc to xrealloc. This change was made in diff-delta.c too,
although the code can handle an out of memory failure.

This patch reverts this change in diff-delta.c.

Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
Signed-off-by: Junio C Hamano <junkio@cox.net>