It can be useful for fixers to record some information about the
inputs they processed. For example, the `FileFixer` we use in our
server at Google get more detailed error information back from its
formatter tools. We may want to record these errors in our
`FileFixer`'s state so we can then return that to the client.
If a commit needs fixing but its descendant does not, we currently
skip rewriting the descendant, which means it will instead be rebased
when the transactions finished. That usually results in
conflicts. This adds a test case for that case.
* The lib part should not deal with tools, or tool config, or running stuff in subprocesses. That stays in the CLI.
* Adds a fix_files function. This is the entry point.
* Adds a FileFixer trait with a single method to process a set of files. This is a high-level plugin point.
* Adds a ParallelFileFixer which implements the FileFixer trait and invokes a function to fix one file a time, in parallel.
* The CLI uses ParallelFileFixer.
The FileFixer trait allows environments (other than jj CLI) to plug in their own implementation; for example, a server could implement a FileFixer that sends RPCs to a "formatting service".
