These changes correct a corner case that the old code managed correctly.

These changes correct a corner case that the old code managed correctly.
Precisely when you have a buffer visiting a file in the root directory
and then revert it to a file with the same name in a different
directory.

(uniquify-fix-item-proposed): Renamed from
uniquify-fix-item-min-proposed.
(uniquify-set-proposed): New function.
(uniquify-rationalize-file-buffer-names): Code reshuffled for
clarity and speed.
(uniquify-item-greaterp): Substitutes uniquify-item-lessp.  This
is equivalent to what the old code did.
(uniquify-rationalize-a-list): Never recompute the proposed
name.  Sort the conflicting sublist before rationalising it: this
is equivalent to what the old code did, but one directory element
at a time, and only when necessary.
(uniquify-rationalize-conflicting-sublist): Recompute here the
proposed name.