Make the rest of commands work from a subdirectory.

These commands are converted to run from a subdirectory.

    commit-tree convert-objects merge-base merge-index mktag
    pack-objects pack-redundant prune-packed read-tree tar-tree
    unpack-file unpack-objects update-server-info write-tree

Signed-off-by: Junio C Hamano <junkio@cox.net>

merge-index.c

@@ -102,6 +102,7 @@ int main(int argc, char **argv)
 	if (argc < 3)
 		usage("git-merge-index [-o] [-q] <merge-program> (-a | <filename>*)");
 
+	setup_git_directory();
 	read_cache();
 
 	i = 1;