watchman-make: avoid dispatching for state-enter changes

Summary:
state-enter and state-leave will wake up subscriptions and
deliver a response that has no files listed.   These events are
triggered in the FB source control deployment when mercurial
transactions are initiated and completed, so these occur around
eg: `commit` and `amend`, and then shortly afterwards when our
background `commit cloud` sync process kicks in.

`watchman-make` was blindly assuming that any wakeup from the
subscription channel was worth running the associated command,
which wasn't true in this situation.

In addition to state enter/leave, watch cancellations can also
deliver a response with an empty file list.  Rather than trying
to continue, let's terminate the process if that happens.

Reviewed By: xixixao

Differential Revision: D19222234

fbshipit-source-id: 06d7ddd9d061f7a21b93b8e4d1d79d3636e9998d

python/bin/watchman-make

@@ -64,7 +64,14 @@ class Target(object):
         data = client.getSubscription(self.name)
         if data is None:
             return
-        self.triggered = True
+        for item in data:
+            # We only want to trigger if files matched;
+            # updates without a files list are metadata
+            # such as state-enter/leave notices so we skip them
+            if "files" in item:
+                self.triggered = True
+            if "canceled" in item:
+                raise RuntimeError("Watch was cancelled")
 
     def execute(self):
         if not self.triggered: