cat: stop using makefileobj() Prepares for porting to the formatter API. We won't be able to utilize the abstraction provided by makefilename() because formatter must be instantiated per file.

formatter: add nullformatter This can be used as a placeholder variable.

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.

formatter: add option to redirect output to file object Commands like 'export' have --output=OUTFILESPEC option, so we need a way to write formatter output optionally to a file.