The code in Keg.self.for path uses "path = path.parent.realpath" to walk
up subfolders looking for a Keg.

Because 'realpath' is in there, and the path is checked against
HOMEBREW_CELLAR, which  may be a symlink, we need to do realpath-to-realpath
comparisons in Keg. Otherwise, we will hit equivalent but symlinked folders,
not see that they are the same, and walk all the way up to / and then
error out.