R1:f7a1feea3867

Begin making change parsers testable

Summary:
Ref T4327. There are a bunch of other probably-related tasks too, some linked there.

We have some rare/unusual bugs in the change parsers, mostly in Subversion, but it's terrifying to touch them because they're complicated and fragile and have no test coverage.

To fix this stuff, I want to make them more testable. In particular, they basically end with this big INSERT right now. Instead, I'm going to make them return objects representing the data to be…