[RFC] vim-patch:7.4.173

Merging vim-patch:7.4.173

Problem:    When using scrollbind the cursor can end up below the last line.
            (mvxxc)
Solution:   Reset w_botfill when scrolling up. (Christian Brabandt)

https://code.google.com/p/vim/source/detail?r=233ad7b960d0fbeb224b383918113b25c74ebe35