fast-export: drop const when storing anonymized values

We store anonymized values as pointers to "const char *", since they are conceptually const to callers who use them. But they are actually allocated strings whose memory is owned by the struct. The ownership mismatch hasn't been a big deal since we never free() them (they are held until the program ends), but let's switch them to "char *" in preparation for changing that. Since most code only accesses them via anonymize_str(), it can continue to narrow them to "const char *" in its return value. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>