[RFC]vim-patch:7.4.309

updated for version 7.4.309
Problem:    When increasing the size of the lower window, the upper window
            jumps back to the top. (Ron Aaron)
Solution:   Change setting the topline. (Nobuhiro Takasaki)

https://code.google.com/p/vim/source/detail?r=88a6e9f33822d33b6c32db578750c6c178c63f50
@justinmk @epmatsw I think 7.4.315 adds a test case for `topline`. Maybe it should be included in this PR.