convert: refactor sink initialisation, to remove hardcoding of hg We also introduce options to explicitly set the source and destination repository types. Improve testing of corner cases a little.

convert: add default constructor for converter_sink

convert: make contents of "extra" dict available from sources, for sinks. This breaks hash preservation for hg->hg conversion, as each converted change gets a convert_revision item added to its extra dict. Ugh.