cache-tree.c: typefix

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/cache-tree.c b/cache-tree.c
index a10902f..28b78f8 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -341,7 +341,7 @@
 	}
 
 	if (dryrun) {
-		char hdr[200];
+		unsigned char hdr[200];
 		int hdrlen;
 		write_sha1_file_prepare(buffer, offset, tree_type, it->sha1,
 					hdr, &hdrlen);