vc-find-root does not work for non-existent file argument

[Dan Nicolaescu]

If FILE that is managed using either hg or bzr (both of which use
vc-find-root) is deleted, doing C-x C-f FILE RET after that will not
result emacs asking the question: "File FILE was lost; check out from version 
control? "

The reason is that (vc-find-root FILE BLAH) returns nil, so VC thinks
that FILE is not under version control. 
vc-find-root returns nil because of the way the result of
file-attributes is used...