This allows you to do
git diff v2.6.12..v2.6.13-rc1 drivers/pcmcia
to see the diff between v2.6.12 and v2.6.13-rc1 as limited by the
filename argument.
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.
Fix permissions, and add trivial "reset" and "add" scripts.
The "reset" script just resets the index back to head, while the "add"
script is just a crutch for people used to do "cvs add".
