vim-patch:7.4.280  #745

Problem:    When using a session file the relative position of the cursor is
            not restored if there is another tab. (Nobuhiro Takasaki)
Solution:   Update w_wrow before calculating the fraction.

https://code.google.com/p/vim/source/detail?r=daf7e98675cf395e1ef96f8040567affb2782a11