fix xcalloc() argument order

fix xcalloc() argument order

Pass the number of elements first and ther size second, as expected
by xcalloc().  Provide a semantic patch, which was actually used to
generate the rest of this patch.

The semantic patch would generate flip-flop diffs if both arguments
are sizeofs.  We don't have such a case, and it's hard to imagine
the usefulness of such an allocation.  If it ever occurs then we
could deal with it by duplicating the rule in the semantic patch to
make it cancel itself out, or we could change the code to use
CALLOC_ARRAY.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>