worker: check problem state correctly (issue3982)

If a large update triggered an abort, it was possible for the main
thread to still update the dirstate.

This fix is incomplete, as the failing worker now doesn't generate a
proper error message. This is difficult in the fork-based framework,
which relies on exceptions propagating to the top of the dispatcher
for formatting.

mercurial/worker.py

@@ -109,7 +109,7 b' def _posixworker(ui, func, staticargs, a'
     def waitforworkers():
         for _ in pids:
             st = _exitstatus(os.wait()[1])
-            if st and not problem:
+            if st and not problem[0]:
                 problem[0] = st
                 killworkers()
     t = threading.Thread(target=waitforworkers)