git's rev-parse.c function show_datestring presumes gnu date

git-rev-parse calls GNU date externally for --since, --after, --before,
and --until, which will fail on at least OpenBSD and Mac OSX (Darwin).

At a minimum, this code can be commented out on such platforms.  Better
would be to have some sort of suitable replacement.

Perhaps the Makefile can test if "date" is GNU date as well.

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