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.