t5000: extract nongit function to test-lib-functions.sh

t5000: extract nongit function to test-lib-functions.sh

This function abstracts the idea of running a command
outside of any repository (which is slightly awkward to do
because even if you make a non-repo directory, git may keep
walking up outside of the trash directory). There are
several scripts that use the same technique, so let's make
the function available for everyone.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>