indexer: kill git_indexer

This was the first implementation and its goal was simply to have
something that worked. It is slow and now it's just taking up
space. Remove it and switch the one known usage to use the streaming
indexer.

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Eventually we'd want to rename `git_indexer_stream` to `git_indexer` but let's go one step at a time. I'm trying to speed up indexing when we have a lot of `REF_DELTA` objects and this is just getting in the way.

As an added bonus, this fixed an uninitialised variable usage that can happen if you don't zero out the stats struct you give to the indexer.