Suppress subcommand output by default (#26)

For large or busy repositories, stack-pr commands can generate hundreds
of lines of output from Git subcommands being executed. Most of the time
these don't matter; the user doesn't care about them. This PR suppresses
them by default by redirecting stdout and stderr to
`subprocess.DEVNULL`. If this output is desired in a specific case, the
command-line flag `--verbose` (`-V`) can be used to avoid this
redirection.