[PATCH 0/3] Remove more parsers

I found some more open-coded tree parsers and a commit parser. These changes shouldn't affect behavior meaningfully, and all of the tests pass. I did notice, however, that the rev-list functionality of pruning by path isn't actually tested in the tests; I forgot to update it and it didn't cause any tests to fail. I also couldn't figure out if "same_tree_as_empty" has important side effects, so I updated its use of diff_tree; in order to do what the name says, "return t1 && !t1->entries" would suffice.

There's also a parser in convert-objects, but it may be best to keep it, since it may want to deal with things which are now considered invalid, and would be rejected by the normal parser.

	-Daniel
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