patman: Use -c to show patch comments

patman: Use -c to show patch comments

We chose -C as the flag to enable showing comments because -c was used
in 'patman send' to specify the patch count. Now that the 'send' parser
is separated we don't need to do this and it is OK to use the same flag
in a different subcommand.

We want to have a flag for cover-letter comments, so it makes most sense
to have -C for that and -c for patch comments. Update the latter.

Put this in a function since the new 'series' command will add this flag
too.

Signed-off-by: Simon Glass <sjg@chromium.org>