hghave: avoid a deadlock reading the child process's output

The output of `pylint` is voluminous enough that it fills the buffer on Windows,
and waited for the parent to read it out.  But the parent was waiting on the
child to exit.

I'm not sure what the intent of setting `ret = -1` in the exception handler just
above this was...

tests/hghave.py

@@ -98,8 +98,8 b' def matchoutput(cmd, regexp, ignorestatu'
         if e.errno != errno.ENOENT:
             raise
         ret = -1
-    ret = p.wait()
-    s = p.stdout.read()
+    s = p.communicate()[0]
+    ret = p.returncode
     return (ignorestatus or not ret) and r.search(s)
 
 @check("baz", "GNU Arch baz client")