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.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.