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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif