Re: [RFC PATCH 1/1] Define an extended tree format

So it all works as expected, but I feel like it's mostly by accident. My
gut feeling is that we probably wanted something like this to protect us
from confusion:

index 06ad74db22..295b0c6318 100644

--- a/read-cache.c
+++ b/read-cache.c

@@ -719,6 +719,8 @@ int add_to_index(struct index_state *istate, const char *path, struct stat *st,
 	if (S_ISDIR(st_mode)) {
 		if (repo_resolve_gitlink_ref(the_repository, path, "HEAD", &oid) < 0)
 			return error(_("'%s' does not have a commit checked out"), path);
+		if (oid.algo != hash_algo_by_ptr(the_repository->hash_algo))
+			return error(_("object format %s of '%s' is incompatible with this repository (%s)"), hash_algos[oid.algo].name, path, the_repository->hash_algo->name);
 		while (namelen && path[namelen-1] == '/')
 			namelen--;
 	}

Of course that is strictly worse for somebody who is relying on the
current accidental behavior. ;) And in the long run, I think this is the
spot we'd want to hook to do whatever massaging we need (whether
converting to the equivalent in-repo algorithm, or hacking up the name
to store the foreign hash).