upspin permission system messes up git diffs when mounted over upspinfs

[filmil]

Not sure how much of an interest there is for git to be fully functional on an upspinfs mount, but in case there is some, here goes.

I noticed that the upspinfs permission system interacts badly with the git diffs. Namely, on many occasions, if the git repo contains permissions that can't be modeled in upspin, checking out a git repo to the upspin filesystem results in a set of bogus permission diffs.

This issue is kind of similar: #331; except in the git case I think bogus diffs are things one really does not want.