rust-dirstate: use EntryState enum instead of literals
This improves code readability quite a bit, while also adding a layer of
safety because we're checking the state byte against the enum.
Differential Revision: https://phab.mercurial-scm.org/D6629
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL