workingctx status calls

[Dirkjan Ochtman]

In 98bc3e195720, I fixed another case where status is also walking
unknown files (in identify). Apparently workingctx() has an unknown()
method, so in order to only ever (lazily) do a single status it needs
to call status with unknown=True. There's only one user for the
unknown() method, in merge, but apparently that method is pretty
ctx-centric.

Any clue about what we should do here?

Cheers,

Dirkjan