vim-patch: 7.4.381

Problem:    Get u_undo error when backspacing in Insert mode deletes more than
            one line break. (Ayberk Ozgur)
Solution:   Also decrement Insstart.lnum.

https://code.google.com/p/vim/source/detail?r=v7-4-381
Merge pull request #1196 from Shougo/vim-patch-7.4.381

vim-patch: 7.4.381