This amends commit f8be71ac6df5dc17a9bb67662c59b01da7a46884.
This still should resolve FS-4303.
What's going on here is that we need a portable way to access
strftime. date(1posix) doesn't provide enough. And without perl, I
can't think of a better way to get to it than just using C. So the
logic for generating the extended revision has been moved into a small
self-contained and hopefully portable C program.
2c634751cc