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

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>