magit-apply: don't always use --3way

With this argument changes are applied to the index in addition to
the working tree, which usually is not desirable.  And if the file
is question modified in the working tree, then it will cause `git
apply' to always fail.

With a prefix argument still use `--3way', but remove the option
`magit-apply-tree-way'.
magit-stash-save: avoid creating an empty "untracked" commit
fix several stashing bugs
magit-stash-create: inhibit commit signing
Merge branch 'n/fix-stash' into next