cmd/upspinfs: fix Rename
cmd/upspinfs: fix Rename

On Rename, at least one FUSE version expects any active node
refering to the old name to now refer to the new one. This
change implements that behavior.

Fixes #580

Change-Id: I9fc4872546518ba08999889969600a90c2fac57b
Reviewed-on: https://upspin-review.googlesource.com/18900
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>

upspin/upspin,upspin/upspin