log: pack filematcher and hunksfilter into changesetdiffer object
This is just a way of getting rid of clumsy makefilematcher/makehunksfilter
arguments. There might be a better abstraction, but I don't think this is bad.
This makes filematcher and hunksfilter available by default, but that should
be fine.
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273ce12ad8f1` will report some style violations on a very old commit).