Clone lists before modifying

Clone lists before modifying

When an opcode expands to a list of jobs, we extend the reason trail
of the new jobs with that of the original opcode that expanded to them.
Before modifying the reason trail, however, we should duplicate it to
avoid side effects on shared copies---like the default empty list.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Lisa Velden <velden@google.com>