test: avoid pipes in git related commands for test

test: avoid pipes in git related commands for test

Avoid using pipes downstream of Git commands since the exit codes
of commands upstream of pipes get swallowed, thus potentially
hiding failure of those commands. Instead, capture Git command
output to a file and apply the downstream command(s) to that file.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>