Reduce backupdir collisions

Vim 'directory' and 'undodir' can assist in preventing collisions by
using a name that consists of the entire path if the setting has a
double slash at the end.  'backupdir' does not have this option, so I am
faking is, well sort of.

Thanks to Doug:

https://groups.google.com/d/msg/vim_use/qJKm5StUvR4/51vdHkyRacUJ