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
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