[PATCH 1 of 2] subrepo: use unset instead of env -u to fix test on BSDs (issue5229)

[Kevin Bullock]

> On May 6, 2016, at 10:36, timeless <timeless at gmail.com> wrote:
> 
> https://www.gnu.org/software/autoconf/manual/autoconf-2.61/html_node/Limitations-of-Builtins.html#Limitations-of-Builtins
> 
> The correct approach is to do construct a subsetted env and use env -i.

It looks like that's only an issue with really ancient shells (Bash 2, others unspecified). I'm inclined to say we don't care if someone sees a failure running the test suite on FreeBSD 4 or something.

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Kevin R. Bullock