Note that this bug is mutually exclusive with issue1534.
It'd be a good idea to refer to msg1579 from issue428.
The problem is that changes --context is ill-defined with matchers like -p and
--last. One solution (issue1534) would be to refuse to accept matchers.
Another solution would be to define a clear meaning for changes --context -p
(etc) and then implement it.
I'll leave this need-action for somebody to work out a proposal.
(One proposal: the matchers mean 'give me a context which includes at least