log: when --cc is given, default to -p unless told otherwise

log: when --cc is given, default to -p unless told otherwise

The "--cc" option to "git log" is clearly a request to show some
sort of combined diff (be it --patch or --raw), but traditionally
we required the command line to explicitly ask for "git log -p --cc".

Teach the command line parser to treat a lone "--cc" as if the user
specified "-p --cc".  Formats that do ask for other forms of diff
output, e.g. "log --raw --cc", are not overriden.

Signed-off-by: Junio C Hamano <gitster@pobox.com>