Allow callers of unpack_trees() to handle failure

Return an error from unpack_trees() instead of calling die(), and exit
with an error in read-tree, builtin-commit, and diff-lib. merge-recursive
already expected an error return from unpack_trees, so it doesn't need to
be changed. The merge function can return negative to abort.

This will be used in builtin-checkout -m.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>

builtin-read-tree.c

@@ -268,7 +268,8 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		parse_tree(tree);
 		init_tree_desc(t+i, tree->buffer, tree->size);
 	}
-	unpack_trees(nr_trees, t, &opts);
+	if (unpack_trees(nr_trees, t, &opts))
+		return 128;
 
 	/*
 	 * When reading only one tree (either the most basic form,