workingctx: build _manifest on filenode() or flags() request
I'm not sure if this is the best workaround, but this fixes the following
exception:
AttributeError: 'workingctx' object has no attribute '_manifestdelta'
The short hash '303030303030' seen in the test is node.modifiednodeid.
Differential Revision: https://phab.mercurial-scm.org/D2940
{header}
# HG graph
# Node ID {node}
# Rows shown {rows}
{nodes%graphnode}