linux requires running `watchman watch` on sourcegraph directory

Created by: beyang

Any other Linux users find they have to run watchman watch path/to/sourcegraph for watchman to work properly? Otherwise, the subscribe invocation results in a "unable to resolve root" error (presumably because the root has to be watched first).

Based on my (limited) understanding of watchman behavior, this ought to be required on macOS, too, but maybe it's obvious to others that they need to run watchman watch separately first?