fast-export: use a flex array to store anonymized entries

fast-export: use a flex array to store anonymized entries

Now that we're using a separate keydata struct for hash lookups, we have
more flexibility in how we allocate anonymized_entry structs. Let's push
the "orig" key into a flex member within the struct. That should save us
a few bytes of memory per entry (a pointer plus any malloc overhead),
and may make lookups a little faster (since it's one less pointer to
chase in the comparison function).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>