Fix `GIT_CEILING_DIRECTORIES` with `C:\` and the likes

When determining the length of the longest ancestor of a given path with respect to to e.g. `GIT_CEILING_DIRECTORIES`, we special-case the root directory by returning 0 (i.e. we pretend that the path `/` does not end in a slash by virtually stripping it). That is the correct behavior because when normalizing paths, the root directory is special: all other directory paths have their trailing slash stripped, but not the root directory's path (because it would become the empty string, which is not a legal path). However, this special-casing of the root directory in `longest_ancestor_length()` completely forgets about Windows-style root directories, e.g. `C:\`. These _also_ get normalized with a trailing slash (because `C:` would actually refer to the current directory on that drive, not necessarily to its root directory). In fc56c7b34b (mingw: accomodate t0060-path-utils for MSYS2, 2016-01-27), we almost got it right. We noticed that `longest_ancestor_length()` expects a slash _after_ the matched prefix, and if the prefix already ends in a slash, the normalized path won't ever match and -1 is returned. But then that commit went astray: The correct fix is not to adjust the _tests_ to expect an incorrect -1 when that function is fed a prefix that ends in a slash, but instead to treat such a prefix as if the trailing slash had been removed. Likewise, that function needs to handle the case where it is fed a path that ends in a slash (not only a prefix that ends in a slash): if it matches the prefix (plus trailing slash), we still need to verify that the path does not end there, otherwise the prefix is not actually an ancestor of the path but identical to it (and we need to return -1 in that case). With these two adjustments, we no longer need to play games in t0060 where we only add `$rootoff` if the passed prefix is different from the MSYS2 pseudo root, instead we also add it for the MSYS2 pseudo root itself. We do have to be careful to skip that logic entirely for Windows paths, though, because they do are not subject to that MSYS2 pseudo root treatment. This patch fixes the scenario where a user has set `GIT_CEILING_DIRECTORIES=C:\`, which would be ignored otherwise. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>