correct cache_entry allocation

Most cache_entry structs are allocated by using the
cache_entry_size macro, which rounds the size of the struct
up to the nearest multiple of 8 bytes (presumably to avoid
memory fragmentation).

There is one exception: the special "conflict entry" is
allocated with an empty name, and so is explicitly given
just one extra byte to hold the NUL.

However, later code doesn't realize that this particular
struct has been allocated differently, and happily tries
reading and copying it based on the ce_size macro, which
assumes the 8-byte alignment.

This can lead to reading uninitalized data, though since
that data is simply padding, there shouldn't be any problem
as a result. Still, it makes sense to hold the padding
assumption so as not to surprise later maintainers.

This fixes valgrind errors in t1005, t3030, t4002, and
t4114.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -382,7 +382,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	o->merge_size = len;
 
 	if (!dfc)
-		dfc = xcalloc(1, sizeof(struct cache_entry) + 1);
+		dfc = xcalloc(1, cache_entry_size(0));
 	o->df_conflict_entry = dfc;
 
 	if (len) {