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
<entry>
<title>{bookmark|escape}</title>
<link rel="alternate" href="{urlbase}{url|urlescape}rev/{node|short}"/>
<id>{urlbase}{url|urlescape}#bookmark-{node}</id>
<updated>{date|rfc3339date}</updated>
<published>{date|rfc3339date}</published>
<content type="text">{bookmark|strip|escape}</content>
</entry>