read-tree() and unpack_trees(): use consistent limit

read-tree -m can read up to MAX_TREES, which was arbitrarily set to 8 since
August 2007 (4 is needed to deal with 2 merge-base case).

However, the updated unpack_trees() code had an advertised limit of 4
(which it enforced).  In reality the code was prepared to take only 3
trees and giving 4 caused it to stomp on its stack.  Rename the MAX_TREES
constant to MAX_UNPACK_TREES, move it to the unpack-trees.h common header
file, and use it from both places to avoid future confusion.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-read-tree.c

@@ -13,16 +13,15 @@
 #include "dir.h"
 #include "builtin.h"
 
-#define MAX_TREES 8
 static int nr_trees;
-static struct tree *trees[MAX_TREES];
+static struct tree *trees[MAX_UNPACK_TREES];
 
 static int list_tree(unsigned char *sha1)
 {
 	struct tree *tree;
 
-	if (nr_trees >= MAX_TREES)
-		die("I cannot read more than %d trees", MAX_TREES);
+	if (nr_trees >= MAX_UNPACK_TREES)
+		die("I cannot read more than %d trees", MAX_UNPACK_TREES);
 	tree = parse_tree_indirect(sha1);
 	if (!tree)
 		return -1;
@@ -97,7 +96,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 {
 	int i, newfd, stage = 0;
 	unsigned char sha1[20];
-	struct tree_desc t[MAX_TREES];
+	struct tree_desc t[MAX_UNPACK_TREES];
 	struct unpack_trees_options opts;
 
 	memset(&opts, 0, sizeof(opts));

unpack-trees.c

@@ -123,7 +123,7 @@ static int unpack_index_entry(struct cache_entry *ce, struct unpack_trees_option
 int traverse_trees_recursive(int n, unsigned long dirmask, unsigned long df_conflicts, struct name_entry *names, struct traverse_info *info)
 {
 	int i;
-	struct tree_desc t[3];
+	struct tree_desc t[MAX_UNPACK_TREES];
 	struct traverse_info newinfo;
 	struct name_entry *p;
 
@@ -327,8 +327,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 {
 	static struct cache_entry *dfc;
 
-	if (len > 4)
-		die("unpack_trees takes at most four trees");
+	if (len > MAX_UNPACK_TREES)
+		die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);
 	memset(&state, 0, sizeof(state));
 	state.base_dir = "";
 	state.force = 1;

unpack-trees.h

@@ -1,6 +1,8 @@
 #ifndef UNPACK_TREES_H
 #define UNPACK_TREES_H
 
+#define MAX_UNPACK_TREES 8
+
 struct unpack_trees_options;
 
 typedef int (*merge_fn_t)(struct cache_entry **src,