fsmonitor: add a sample integration script for Watchman

This script integrates the new fsmonitor capabilities of git with the
cross platform Watchman file watching service. To use the script:

Download and install Watchman from https://facebook.github.io/watchman/.
Rename the sample integration hook from fsmonitor-watchman.sample to
fsmonitor-watchman. Configure git to use the extension:

git config core.fsmonitor .git/hooks/fsmonitor-watchman

Optionally turn on the untracked cache for optimal performance.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>