rerere: check dirname format while iterating rr_cache directory

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>