mq: regularize patch header creation.
Do not prefix qnew patches with [mq]: when a message has been given.
Always use [mq]: as the prefix for generated commit messages.
adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2
adding a
adding c
adding b
adding d
removing a
removing c
recording removal of a as rename to b (100% similar)