Fix read-tree merging more than 3 trees using 3-way merge

For multi-base merges, we allowed read-tree -m to take more than
three trees (the last two are our and their branches, and all the
earlier ones, typically one but potentially more, are used as the
merge base).  Unfortunately, the conversion done by commit 933bf40
broke this.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-read-tree.c

@@ -13,8 +13,9 @@
 #include "dir.h"
 #include "builtin.h"
 
+#define MAX_TREES 4
 static int nr_trees;
-static struct tree *trees[4];
+static struct tree *trees[MAX_TREES];
 
 static int list_tree(unsigned char *sha1)
 {
@@ -96,7 +97,7 @@ 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[3];
+	struct tree_desc t[MAX_TREES];
 	struct unpack_trees_options opts;
 
 	memset(&opts, 0, sizeof(opts));
@@ -263,6 +264,9 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 			opts.head_idx = 1;
 	}
 
+	if (MAX_TREES < nr_trees)
+		die("I cannot read more than %d trees", MAX_TREES);
+
 	for (i = 0; i < nr_trees; i++) {
 		struct tree *tree = trees[i];
 		parse_tree(tree);