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