fix: allow fixer tools to return metadata in addition to the file content With this change, fixer tools can be configured to output a JSON object that will be parsed and passed to hooks that can be used to print summaries of what code was formatted or perform other post-fixing work. The motivation for this change is to allow parallel executions of a "meta-formatter" tool to report back statistics, which are then aggregated and processed after all formatting has completed. Providing an extensible mechanism inside fix.py is far simpler, and more portable, than trying to make a tool like this communicate through some other channel. Differential Revision: https://phab.mercurial-scm.org/D6167