narrow: drop explicit dirstate write
The dirstate is written when the wlock is released, so we don't need
to write it explicitly in updateworkingcopy(). I don't know why I put
it there in the first place (tests pass without it even in the commit
that introduced it).
Differential Revision: https://phab.mercurial-scm.org/D5515
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL