fsmonitor: coerce `clock` variable to byte-string (issue6321) Callers of `fsmonitor.state.setlastclock` pass their arguments wrapped in `pycompat.sysbytes` to ensure the value is a `bytes` on Python 3. However in `fsmonitor.poststatus.__call__`, if the return value of `getlastclock()` is `None`, we use the value of `fsmonitor.poststatus._startclock` instead, which is not converted to a byte string in the same manner. This commit converts the value of `startclock` to a byte string using `pycompat.sysbytes` in the constructor for `poststatus`, to avoid the "`str` + `bytes`" error from issue 6321. Differential Revision: https://phab.mercurial-scm.org/D8573