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
Merge pull request #12 from purcell/double-fullframe-fix

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