magit-branch-delete: force delete with prefix again

This fixes 2db4566c3f55d8621731dfc9e54f575c05b0db34 which accidentally
removed the option to force deletion of unmerged branches by using a
prefix argument.
magit-branch-delete: cosmetics
magit-backup-mode: enable by default again

This might be causing performance issues on large repositories and
we might disable this again before the release, but currently a lot
of things are changing rapidly and it's a good idea to have reliable
backups.
magit-diff-wash-revision-insert-related-refs: new variable

This is only intended as a temporary workaround for
https://github.com/magit/magit/issues/1529#issuecomment-61178078.
magit-{modified,untracked}-files: bugfix

`git ls-files' does not support `--ignore-submodules' so in
`magit-modified-files' use `git diff-files' instead and remove
`magit-untracked-files' NOMODULES argument.
magit-insert-rebase-sequence: bugfix
temporarily disable commands magit-stash-{index,worktree}

They eat data.
magit-diff-type: bugfix
magit-branch-set-upstream: bugfix

Cannot use `magit-read-local-branch' because user has to be
able to _not_ select a branch in order to _unset_ upstream.
magit-push-tags: cosmetics
magit-pull: fix doc-string
magit-copy-as-kill: use full hashes
magit-diff-{,refresh-}popup: fix typo
magit-branch-set-upstream: bugfix
magit-request-pull: don't prompt for branch

Because we don't actually use the selected branch.  We should
actually allow picking a branch but that will come later.
magit-clone: cosmetics
magit-clone: create parent directories of destination
magit-remote-{add,remove,rename}: improve doc-strings

Remove Git usage (that's not really useful) and improve wording.
magit-remote-remove: move definition
magit-remote-update: add ARGS argument