mktree: fix a memory leak in write_tree()

We forget to call strbuf_release to release the buf memory.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mktree.c

@@ -61,6 +61,7 @@ static void write_tree(unsigned char *sha1)
 	}
 
 	write_sha1_file(buf.buf, buf.len, tree_type, sha1);
+	strbuf_release(&buf);
 }
 
 static const char mktree_usage[] = "git-mktree [-z]";