fast-export: factor out anonymized_entry creation
When anonymizing output, there's only one spot where we generate new
entries to add to our hashmap: when anonymize_str() doesn't find an
entry, we use the generate() callback to make one and add it. Let's pull
that into its own function in preparation for another caller.
Note that we'll add one extra feature. In anonymize_str(), we know that
we won't find an existing entry in the hashmap (since it will only try
to add after failing to find one). But other callers won't have the same
behavior, so we should catch this case and free the now-dangling entry.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>