The high level changes include:
- Reworking `fix_file_ids()` to loop over multiple candidate tools per file,
piping file content between them. Only the final file content is written to
the store, and content is no longer read for changed files that don't match
any of the configured patterns.
- New struct `ToolsConfig` to represent the parsed/validated configuration.
- New function `get_tools_config()` to create a `ToolsConfig` from a `Config`.
- New tests; the only old behavior that has changed is that we don't require
`fix.tool-command` if `fix.tools` defines one or more tools. The general
approach to validating the config is to fail early if anything is weird.
Co-Authored-By: Josh Steadmon <steadmon@google.com>
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