fsmonitor: don't attempt state-leave if we didn't state-enter The state-enter command may not have been successful; for example, the watchman client session may have timed out if the user was busy/idle for a long period during a merge conflict resolution earlier in processing a rebase for a stack of diffs. It's cleaner (from the perspective of the watchman logs) to avoid issuing the state-leave command in these cases. Test Plan: ran `hg rebase --tool :merge -r '(draft() & date(-14)) - master::' -d master` and didn't observe any errors in the watchman logs or in the output from `watchman -p -j <<<'["subscribe", "/data/users/wez/fbsource", "wez", {"expression": ["name", ".hg/updatestate"]}]'`