streamclone: add support for cloning non append-only file
The phaseroots are stored in a non append-only file in the repository. We
include them in the stream too. Since they are not append-only, we have to
keep a copy around while we hold the lock to be able to stream them later.
Since phase get exchanged within the stream we can skip requesting them
independently.
As a side effect, this will fixes
issue5648 once the feature is enabled by
default.