Stop annoying backup-fail messages on Windoze/gvim.
(Yes, this turns off backups for windows.)
More backup file tweaks, also .gitignore.
Backup is back on Windows/gvim.
It is now able to automatically create a backup directory on Windows.
It creates this directory in the dotvim directory in this git tree,
so it was prudent to make a .gitignore to avoid tracking the backups
and temporary files.  While we're at it, we'll stop tracking session
changes too.