fast-export: store anonymized oids as hex strings

fast-export: store anonymized oids as hex strings

When fast-export stores anonymized oids, it does so as binary strings.
And while the anonymous mapping storage is binary-clean (at least as of
the previous commit), this will become awkward when we start exposing
more of it to the user. In particular, if we allow a method for
retaining token "foo", then users may want to specify a hex oid as such
a token.

Let's just switch to storing the hex strings. The difference in memory
usage is negligible (especially considering how infrequently we'd
generally store an oid compared to, say, path components).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>