[PATCH 0 of 3 V2] Add support for using monotone's "automate stdio" to the convert extension

[daniel.atallah at gmail.com]

(Updated set of patches based on review of the first version)

Add support for using monotone's "automate stdio" to the convert extension

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.