t: use nongit() function where applicable

t: use nongit() function where applicable

Many tests want to run a command outside of any git repo;
with the nongit() function this is now a one-liner. It saves
a few lines, but more importantly, it's immediately obvious
what the code is trying to accomplish.

This doesn't convert every such case in the test suite; it
just covers those that want to do a one-off command. Other
cases, such as the ones in t4035, are part of a larger
scheme of outside-repo files, and it's less confusing for
them to stay consistent with the surrounding tests.

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