Use "-M" instead of "-C" for "git diff" and "git status"

Use "-M" instead of "-C" for "git diff" and "git status"

The "C" in "-C" may stand for "Cool", but it's also pretty slow, since
right now it leaves all unmodified files to be tested even if there are
no new files at all.  That just ends up being unacceptably slow for big
projects, especially if it's not all in the cache.