git-cvsimport-3: don't execute subcommands with the shell

Creating command strings to execute subcommands via the shell means that
care needs to be taken to quote any user-supplied values, but Python
provides a better way by passing a list of arguments when creating
subprocesses.

Make this change, changing pipelines that currently use the shell to use
pipelined subprocesses.

Signed-off-by: John Keeping <john@keeping.me.uk>