fsmonitor: refresh pywatchman with upstream This commit vendors pywatchman commit 259dc66dc9591f9b7ce76d0275bb1065f390c9b1 from upstream without modifications. The previously vendored pywatchman from changeset 16f4b341288d was from Git commit c77452. This commit effectively undoes the following Mercurial changesets: * dd35abc409ee fsmonitor: correct an error message * b1f62cd39b5c fsmonitor: layer on another hack in bser.c for os.stat() compat (issue5811) * c31ce080eb75 py3: convert arguments, cwd and env to native strings when spawning subprocess * 876494fd967d cleanup: delete lots of unused local variables * 57264906a996 watchman: add the possibility to set the exact watchman binary location The newly-vendored code has support for specifying the binary location, so 57264906a996 does not need applied. But we do need to modify our code to specify a proper argument name. 876494fd967d is not important, so it will be ignored. c31ce080eb75 globally changed the code base to always pass str to subprocess. But pywatchman's code is Python 3 clean, so we don't need to do this. This leaves dd35abc409ee and b1f62cd39b5c, which will be re-applied in subsequent commits. Differential Revision: https://phab.mercurial-scm.org/D7201