fractional file timestamps in tramp-adb

[Paul Eggert]

The attached recently-installed patchchanges the tramp-adb's generated 
shell command from this format:

touch -t 201902141023.17 file

to this format:

touch -d 2019-02-14T18:23:17 file

A downside of the new approach is that it assumes POSIX.1-2008 or later, 
but I guess that's OK nowadays. An upside is that 'touch -d' lets one 
specify the fractional part of the timestamp, and also lets one specify 
that the timestamp is UTC by appending "Z" (I don't see how the current 
code is arranging for UTC). So, how about the attached followup patch? 
It would change the format to:

touch -d 2019-02-14T18:23:17.318491822Z file

0001-Add-fractional-file-timestamps-to-tramp-adb.patch
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