workingctx: use inheritance for _buildstatus while keeping the fastpath
This patch maintains the fast path for workingctx which is to not build a
manifest if the working directory is being compared to its parent since, in
this case, we can just copy the parent manifest.
localrepo: remove unneeded unpacking of r
This unpacking is unneeded now because previous patches have removed the need
for this method to modify each of these variables in favor of passing the whole
set around to pre/post hook methods of the corresponding context object.