test-prune: Read to the end of cut(1) output

test-prune: Read to the end of cut(1) output

If we use head(1) to take only the first two lines, then cut(1) and
earlier pipeline entries are killed by SIGPIPE (if they have not already
terminated), and that's flagged as an error under `set -o pipefail`.
Use an equivalent sed command to take exactly the second line, but
without SIGPIPE.

Signed-off-by: Simon McVittie <smcv@debian.org>
Gbp-Pq: Name test-prune-Read-to-the-end-of-cut-1-output.patch