git-diff: do not turn off -p/-M with any diff options.

git-diff: do not turn off -p/-M with any diff options.

When the user gives a diff option (e.g. --cached) to "git diff",
we turned off the built-in default option -p, which is usually
not what user wants to see.

This commit makes lack of --name-status, --name-only nor -r to
add -p, and lack of -B*, -C* nor -M* to add -M to the flags
given to the underlying diff.

Signed-off-by: Junio C Hamano <junkio@cox.net>