Fix watchman cppclient test
Fix watchman cppclient test

Summary:
Watchman's cppclient test fails when it encounters updates which aren't related to changes in the actual files, like `state-enter` and `state-leave` updates.

The exception message:

```
terminate called after throwing an instance of 'folly::TypeError'
  what():  TypeError: expected dynamic type `boolean', but had type `null'
```

The stack trace pointed to EventBase.cpp and SignalHandler.cpp, but using GDB to see when the actual `folly::TypeError was constructed` gave a more useful stacktrace pointing at

https://our.intern.facebook.com/intern/diffusion/FBS/browse/master/fbcode/watchman/tests/integration/cppclient.cpp?commit=ef650b6883651846fc48ec22fbcea27b7efb09e5&lines=57

Printing the actual value of `data` showed that this is the update we failed on:

```
{
  "metadata": {
    "rev": "d75ace9d1252557db6a6d52af9811cdce455a4d3",
    "distance": 1,
    "partial": false,
    "status": "ok",
    "merge": false
  },
  "subscription": "sub1",
  "state-leave": "hg.update",
  "clock": "c:1581022870:3947991:2710700034:123067",
  "version": "2020-01-26T13:32:12Z",
  "unilateral": true,
  "root": "/data/users/jhemphill/fbsource"
}
```

I think checking for the `"files"` field gives us a good mix of keeping the test simple and screening for the kind of update we're interested in here.

Reviewed By: wez

Differential Revision: D20075421

fbshipit-source-id: 9154eb9b0446b1453183f785dcb940fec4c46c14

facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman