Make "cat-file" use "read_object_with_reference()"

This means that you can take a tag object, and do

	git-cat-file commit tagname

and it will cat the commit that the tag points to. Or you can
cat the tree that a commit (or tag) points to.

It still gives the old behaviour if you just give it the
original type, ie if you want to see the tag object itself,
you'd do

	git-cat-file -t tagname

and you'd get the expected tag output.

cat-file.c

@@ -14,18 +14,21 @@ int main(int argc, char **argv)
 
 	if (argc != 3 || get_sha1(argv[2], sha1))
 		usage("cat-file [-t | tagname] <sha1>");
-	buf = read_sha1_file(sha1, type, &size);
-	if (!buf)
-		die("cat-file %s: bad file", argv[2]);
+
 	if (!strcmp("-t", argv[1])) {
-		buf = type;
-		size = strlen(type);
-		type[size] = '\n';
-		size++;
-	} else if (strcmp(type, argv[1])) {
-		die("cat-file %s: bad tag", argv[2]);
+		buf = read_sha1_file(sha1, type, &size);
+		if (buf) {
+			buf = type;
+			size = strlen(type);
+			type[size] = '\n';
+		}
+	} else {
+		buf = read_object_with_reference(sha1, argv[1], &size, NULL);
 	}
 
+	if (!buf)
+		die("cat-file %s: bad file", argv[2]);
+
 	while (size > 0) {
 		long ret = write(1, buf, size);
 		if (ret < 0) {