get-tar-commit-id: use TYPEFLAG_GLOBAL_HEADER instead of magic value

Use the same macro in the archive reader code as on the writer side in
archive-tar.c to document the connection.

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

builtin/get-tar-commit-id.c

@@ -32,7 +32,7 @@ int cmd_get_tar_commit_id(int argc, const char **argv, const char *prefix)
 		die_errno("git get-tar-commit-id: read error");
 	if (n != HEADERSIZE)
 		die_errno("git get-tar-commit-id: EOF before reading tar header");
-	if (header->typeflag[0] != 'g')
+	if (header->typeflag[0] != TYPEFLAG_GLOBAL_HEADER)
 		return 1;
 
 	len = strtol(content, &end, 10);