Detect root directory renames

[nathansobo]

When I rename a directory, Zed doesn't update to reflect the rename.

[notpeter]

I'm still able to reproduce this with Zed 0.142:

mkdir -p /tmp/junk
zed /tmp/junk
mv /tmp/junk /tmp/stuff

Zed.log:

[ERROR] failed to canonicalize root path: No such file or directory (os error 2)