Fix error in resolving a folder as a volume mount point

Assuming you have a repo cloned to the following directory structure:
C:\dir1\dir2\dir3\repo

Lets say that 'C:\dir1\dir2' is actually the mount point for a volume (and this is the only mount point for the volume).

When fetching from a remote, it will end up calling fileops.c::git_futils_mkdir().  This attempts to ascertain if 'C:\dir1\dir2' is a directory or not, which depends on the result from posix_w32.c::l_statw().

In the current implementation, the call to GetFinalPathNameByHandleW would return 'C:\dir1\dir2' and the following call to GetFileAttributesExW in l_statw() would be for the junction point.  This causes git_futils_mkdir() to fail (since S_ISDIR is not set).

This patch modifies getfinalpath_w() to return the full path from the volume root (i.e. \\?\Volume{}\) so GetFileAttributesExW can query for the metadata of the final target.