fast-export: de-obfuscate --anonymize-map handling

When we handle an --anonymize-map option, we parse the orig/anon pair, and then feed the "orig" string to anonymize_str(), along with a generator function that duplicates the "anon" string to be cached in the map. This works, because anonymize_str() says "ah, there is no mapping yet for orig; I'll add one from the generator". But there are some downsides: 1. It's a bit too clever, as it's not obvious what the code is trying to do or why it works. 2. It requires allowing generator functions to take an extra void pointer, which is not something any of the normal callers of anonymize_str() want. 3. It does the wrong thing if the same token is provided twice. When there are conflicting options, like: git fast-export --anonymize \ --anonymize-map=foo:one \ --anonymize-map=foo:two we usually let the second one override the first. But by using anonymize_str(), which has first-one-wins logic, we do the opposite. So instead of relying on anonymize_str(), let's directly add the entry ourselves. We can tweak the tests to show that we handle overridden options correctly now. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>