rerere: check dirname format while iterating rr_cache directory

In rerere_gc(), we walk over the .git/rr_cache directory and create a struct for each entry we find. We feed any name we get from readdir() to find_rerere_dir(), which then calls get_sha1_hex() on it (since we use the binary hash as a lookup key). If that fails (i.e., the directory name is not what we expected), it returns NULL. But the comment in find_rerere_dir() says "BUG". It _would_ be a bug for the call from new_rerere_id_hex(), the only other code path, to fail here; it's generating the hex internally. But the call in rerere_gc() is using it say "is this a plausible directory name". Let's instead have rerere_gc() do its own "is this plausible" check. That has two benefits: - we can now reliably BUG() inside find_rerere_dir(), which would catch bugs in the other code path (and we now will never return NULL from the function, which makes it easier to see that a rerere_id struct will always have a non-NULL "collection" field). - it makes the use of the binary hash an implementation detail of find_rerere_dir(), not known by callers. That will free us up to change it in a future patch. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>