`magit-stage-all': make confirmation optional

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-unstage-all': make confirmation optional

Signed-off-by: Pieter Praet <pieter@praet.org>
magit-tests.el: fix `magit-status-staged-all' test

Commit d02460cd added a confirmation prompt to `magit-{,un}stage-all'
but didn't update the `magit-status-staged-all' test accordingly,
causing it to fail.  As a result, all Travis CI builds are currently
marked as being broken.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #970 from praet/staging-confirmation

Fix Travis build failures due to `magit-stage-all` confirmation