perf-lib: fix ignored exit code inside loop

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>