bug#68808: subsecond mtime discovery code insufficient

[Erik A Johnson]

        I'm guessing the Mac (and its sleep command?) do support subsecond 
mtimes in reality.

echo a > 1 && echo b > 2 && echo c > 3 && ls -t [123]
consistently gives me the correct order (3 2 1).
The macOS version of stat has a mtime_nsec field that is fully populated and 
the file modification times differ by about a half of a millisecond.

        But, looking at that code, I belatedly see that shell arrays are being
        used. At least I think so. It wouldn't surprise me if the mac /bin/sh
        doesn't support arrays. But that should result in assuming subsecond
        mtimes are not supported when they are, instead of the reverse. Anyway,
        I don't think we can assume arrays, so I'll have to think about that.

On macOS, /bin/sh is actually /bin/bash executing in POSIX mode.  I _think_ it 
has arrays but I could check if you have some code you want me to run.