formatter: add helper to create a formatter optionally backed by file To make things simple, openformatter() and maybereopen() have no support for a plain object API. Callers must use the "with" statement. Unlike cmdutil.makefileobj(), append mode ('ab') isn't supported by these functions. This is because JSON output can't be simply concatenated. Perhaps cmdutil.export() will have to build a {filename: [revs...]} map first and write revs per file.