context: override _dirstatestatus in workingcommitctx for correct matching
Before this patch, the result of "status()" on "workingcommitctx" may
incorrectly contain files other than ones to be committed, because
"workingctx._dirstatestatus()" returns the result of
"dirstate.status()" directly.
For correct matching, this patch overrides "_dirstatestatus" in
"workingcommitctx" and makes it return matched files only in
"self._status".
This patch uses empty list for "deleted", "unknown" and "ignored" of
status, because status between "changectx"s also makes them empty.
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