fsmonitor: set the PWD to the top of the working tree

fsmonitor: set the PWD to the top of the working tree

The fsmonitor command inherits the PWD of its caller, which may be
anywhere in the working copy.  This makes is difficult for the
fsmonitor command to operate on the whole repository.  Specifically,
for the watchman integration, this causes each subdirectory to get its
own watch entry.

Set the CWD to the top of the working directory, for consistency.

Signed-off-by: Alex Vandiver <alexmv@dropbox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>