diff-lib, read-tree, unpack-trees: mark cache_entry array paramters const

Change the type merge_fn_t to accept the array of cache_entry pointers
as const pointers to const pointers.  This documents the fact that the
merge functions don't modify the cache_entry contents or replace any of
the pointers in the array.

Only a single cast is necessary in unpack_nondirectories because adding
two const modifiers at once is not allowed in C.  The cast is safe in
that it doesn't mask any modfication; call_unpack_fn only needs the
array for reading.

Signed-off-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-lib.c

@@ -424,7 +424,8 @@ static void do_oneway_diff(struct unpack_trees_options *o,
  * the fairly complex unpack_trees() semantic requirements, including
  * the skipping, the path matching, the type conflict cases etc.
  */
-static int oneway_diff(struct cache_entry **src, struct unpack_trees_options *o)
+static int oneway_diff(const struct cache_entry * const *src,
+		       struct unpack_trees_options *o)
 {
 	const struct cache_entry *idx = src[0];
 	const struct cache_entry *tree = src[1];