Commit b235bde3 dropped the "linenumber" argument from
filectx.annotate(). Since we only ever pass "prefetchskip" by name, we
can safely access it via kwargs.
This patch doesn't update fastannotate in a backwards compatible way
simply because we don't use fastannotate at Google (we do use absorb,
and hopefully that doesn't depend on fastannotate's override?).