patman: Cache the CC list from MakeCcFile() for use in ShowActions()

Currently we go through and generate the CC list for patches twice.
This gets slow when (in a future CL) we add a call to
get_maintainer.pl on Linux.  Instead of doing things twice, just cache
the CC list when it is first generated.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>