convert/mtn: add support for using monotone's "automate stdio" when available Currently the convert extension spawns a new mtn process for each operation. For a large repository, this ends up being hundreds of thousands of processes. The following enables usage of monotone's "automate stdio" functionality - documented at: http://www.monotone.ca/docs/Automation.html#index-mtn-automate-stdio-188 The effect is that (after determining that a new enough mtn executable is available) a single long-running mtn process is used for all the operations, using stdin/stdout to send commands and read output. This has a pretty significant effect on the performance of some parts of the conversion process.