fixup relative_root usage in watchman-wait
fixup relative_root usage in watchman-wait

Summary:
I'm not sure why were weren't using this already; wonder if I'd written this before sid0 added the relative_root handling?

The problem scenario is:

```
cd .hg && watchman-wait . --pattern foo
```

Then, `touch .hg/foo` should cause the process to terminate, but it doesn't

Reviewed By: farnz

Differential Revision: D5074910

fbshipit-source-id: b1a464f3efaa36b42aff5dfd106ee022fe17d5e6

wez/watchman,kwlzn/watchman,nodakai/watchman,kwlzn/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,wez/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,kwlzn/watchman,facebook/watchman,nodakai/watchman,wez/watchman,kwlzn/watchman,facebook/watchman,kwlzn/watchman,wez/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,wez/watchman,nodakai/watchman,facebook/watchman,wez/watchman,wez/watchman