Don't store previous window config if command + full-framing didn't change it

This fixes the following scenario:

- User has: `(fullframe magit-status magit-mode-quit-window)`
- User runs `magit-status` twice in succession
- User runs `magit-mode-quit-window`
- Expected result: the original config should be restored
- Actual result: the buffer remains on-screen, full-frame, as if `magit-mode-quit-window` had no effect