do not enable the backup mode by default

`magit-backup-mode' creates a stash whenever the user applies
(e.g. stages) a change.  This necessarily takes time and is being
perceived as a performance regression.  Do not enable the mode by
default, so that users have to opt-in.

Also no longer include untracked files in the stash by default.
This takes even more time, especially in repositories lacking
proper ignore rules.