rust-dirstate: don't return a state for untracked entries
This `state` API is a remnant of the former API and is slated for removal at
some point. Any caller of this function will expect an entry that is tracked
in the larger sense.
Differential Revision: https://phab.mercurial-scm.org/D12448
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present