OS X's `xargs` does not support the `-r` flag (which means "do not
execute the command even once if there are no arguments"), but
behaves by default as if it was specified.
You can verify this yourself with this test:
$ touch zero-length-file
$ xargs echo <zero-length-file
If `echo` is executed, you will see a blank line. If it is not
executed (i.e. `-r` is specified or the behavior is implied) then
you will see no blank line.
Run remote rsync as root to guarantee that rsync can write to guestpath.
This obviates the need to chown the guestpath to the SSH user prior to
sync.
This brings a substantial speedup (2x on a moderately-sized shared
folder) and properly triggers filesystem notifications on only the files
changed by a given sync.
