convert: use subprocess for all commandline calls
Avoid mixing popen and subprocess calls, it simplifies the command line
generation and quoting issues with redirections.
In practice, it fixes the subversion sink on Windows and probably helps
with monotone and darcs sources.
largefiles: adjust localstore to handle batch statlfile requests (
issue3583)
This fixes a traceback when pushing to a local repo which started with
9e1616307c4c.