diff.c: rearrange xcalloc arguments
diff.c: rearrange xcalloc arguments

xcalloc() takes two arguments: the number of elements and their size.
diffstat_add() passes the arguments in reverse order, passing the
size of a diffstat_file*, followed by the number of diffstat_file* to
be allocated.

Rearrange them so they are in the correct order.

Signed-off-by: Brian Gesiak <6667b8c77ec6ba15b2e807ad7a2130b19e9fa935@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git