#11999: sync based on comparing mtimes, not mtime to system clock

diff --cc Lib/mailbox.py
Simple merge

diff --cc Lib/test/test_mailbox.py
Simple merge

diff --cc Misc/NEWS
index 52d1d61c4443f59deaf9fb135b39565421111055,2e31693975ba39f736ed69013e3508e4bb1a43d9..1f48a14d441f53b48da832e35a94d79a7a1fc78c
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -83,20 -66,13 +83,24 @@@ Core and Builtin
  Library
  -------
  
 -- Issue #10684: shutil.move used to delete a folder on case insensitive
 -  filesystems when the source and destination name where the same except
 -  for the case.
+ - Issue 11999: fixed sporadic sync failure mailbox.Maildir due to its trying to
+   detect mtime changes by comparing to the system clock instead of to the
+   previous value of the mtime.
+ 
 +- ntpath.samefile failed to notice that "a.txt" and "A.TXT" refer to the same
 +  file on Windows XP. As noticed in issue #10684.
 +
 +- Issue #12000: When a SSL certificate has a subjectAltName without any
 +  dNSName entry, ssl.match_hostname() should use the subject's commonName.
 +  Patch by Nicolas Bareil.
 +
 +- Issue #11647: objects created using contextlib.contextmanager now support
 +  more than one call to the function when used as a decorator. Initial patch
 +  by Ysj Ray.
 +
 +- logging: don't define QueueListener if Python has no thread support.
 +
 +- functools.cmp_to_key() now works with collections.Hashable().
  
  - Issue #11277: mmap.mmap() calls fcntl(fd, F_FULLFSYNC) on Mac OS X to get
    around a mmap bug with sparse files. Patch written by Steffen Daode Nurpmeso.