fsmonitor: coerce `clock` variable to byte-string (issue6321)

Authored by sheehan.


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