diff format documentation: clarify --cc and -c

diff format documentation: clarify --cc and -c

The description was unclear if -c or --cc was the default (--cc is for
some commands), and incorrectly implied that the default applies to
all the diff generating commands.

Most importantly, "log" does not default to "--cc" (it defaults to
"--no-merges") and "log -p" obeys the user's wish to see non-combined
format.  Only "diff" (during merge and three-blob comparison) and
"show" use --cc as the default.

Signed-off-by: Adam Monsen <haircut@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>