fsmonitor: add a sample integration script for Watchman

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 <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>