worker: flush messages written by child processes before exit

I found some child outputs were lost while testing the previous patch. Since
os._exit() does nothing special, we need to do that explicitly.

mercurial/worker.py

@@ -148,12 +148,16 def _posixworker(ui, func, staticargs, a
             # may do some clean-ups which could cause surprises like deadlock.
             # see sshpeer.cleanup for example.
             try:
+                try:
                 scmutil.callcatch(ui, workerfunc)
+                finally:
+                    ui.flush()
             except KeyboardInterrupt:
                 os._exit(255)
             except: # never return, therefore no re-raises
                 try:
                     ui.traceback()
+                    ui.flush()
                 finally:
                     os._exit(255)
             else:

tests/test-simple-update.t

@@ -84,7 +84,8 update with worker processes
 
   $ hg update null
   0 files updated, 0 files merged, 100 files removed, 0 files unresolved
-  $ hg update
+  $ hg update -v | grep 100
+  getting 100
   100 files updated, 0 files merged, 0 files removed, 0 files unresolved
 
   $ cd ..