sync: switch to multiprocessing.Event

We've switched most of this command over to multiprocessing and off
of _threading, so do the Event object too.  The APIs are the same
between the modules, so we shouldn't need to update anything else.

Change-Id: I52d31f1c6ef2bcbe7bbc1dd1add79a8d5d08784a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305147
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
diff --git a/subcmds/sync.py b/subcmds/sync.py
index 9a09be6..b50fd94 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -850,7 +850,7 @@
           print('error: failed to remove existing smart sync override manifest: %s' %
                 e, file=sys.stderr)
 
-    err_event = _threading.Event()
+    err_event = multiprocessing.Event()
 
     rp = self.manifest.repoProject
     rp.PreSync()