Avoid file time stamp bug on MS-Windows.
* fileio.c (Fwrite_region): Don't use the heuristic on empty files, as FAT32 doesn't update time stamps when truncating them. Also, check that a file time stamp is not a multiple of 100 ns; this should catch all instances of the problem on MS-Windows, as its native file system resolution is 100 ns or worse, and checking for a non-multiple of 100 ns should impose only a small overhead on systems with ns resolution. Fixes: debbugs:13149
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