[PATCH 3/5] optimize parse_sha1_header() a little by detecting object type

@@ -1099,7 +1099,8 @@ static void *map_sha1_file(const unsigned char *sha1, unsigned long *size)
 
 		if (!fstat(fd, &st)) {
 			*size = xsize_t(st.st_size);
-			map = xmmap(NULL, *size, PROT_READ, MAP_PRIVATE, fd, 0);
+			if (*size > 0)
+				map = xmmap(NULL, *size, PROT_READ, MAP_PRIVATE, fd, 0);
 		}
 		close(fd);
 	}

@@ -1257,6 +1258,8 @@ static int parse_sha1_header(const char *hdr, unsigned long length, unsigned lon
 	 * terminating '\0' that we add), and is followed by
 	 * a space, at least one byte for size, and a '\0'.
 	 */
+	if ('b' != *hdr && 'c' != *hdr && 't' != *hdr)	/* blob/commit/tag/tree */
+		return -1;
 	i = 0;
 	while (hdr < hdr_end - 2) {
 		char c = *hdr++;