perf-lib: fix ignored exit code inside loop

When copying the test repository, we try to detect whether the copy succeeded. However, most of the heavy lifting is done inside a for loop, where our "break" will lose the exit code of the failing "cp". We can take advantage of the fact that we are in a subshell, and just "exit 1" to break out with a code. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>