t6101: avoid suppressing git's exit code

t6101: avoid suppressing git's exit code

Update t6101-rev-parse-parents.sh to redirect git-rev-parse
output to a temporary file instead of piping it directly to
not hide the exit code of git commands behind pipes, as a
crash in git might go unnoticed.

Signed-off-by: Trieu Huynh <vikingtc4@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>