mq: fix opts['unified'] = '0' to take effect when qdiff --stat...
mq: fix opts['unified'] = '0' to take effect when qdiff --stat
`opts['unified'] = '0'` tries to reduce the number of lines to be
processed by patch.diffstat, but here it does not affect diffopts,
thus it does not make sense.
Instead, we can change `diffopts.context` directly.
Added test case for qdiff --stat.