contrib/docker/debian-jessie

context: move _manifest from committablectx to workingctx

committablectx had a _manifest implementation that was only used by the derived
workingctx class. The other derived versions, like memctx and metadataonlyctx,
define their own _manifest functions.

Let's move the function down to workingctx, and let's break it into two parts,
the _manifest part that reads from self._status, and the part that actually
builds the new manifest. This separation will let us reuse the builder code in a
future patch to answer _buildstatus with varying status inputs, since workingctx
has special behavior for _buildstatus that the other ctx's don't have.

FROM debian:jessie
RUN apt-get update && apt-get install -y \
  build-essential \
  debhelper \
  dh-python \
  devscripts \
  python \
  python-all-dev \
  python-docutils \
  zip \
  unzip