Drop support for CtrlP in history

This is fairly straightforward to do outside of fugitive.vim, and
there's no reason to give it privileged treatment here.

Closes #403.
Limit scope of ctrlp.vim :Gstatus hack
Revert "Ridiculous hack to set cursor position on :Gdiff"

This reverts commit c8070aec7e66772322f968a748af757938931b20.

Closes #471.
Restore ridiculous hack to set cursor position on :Gdiff

This time with :setlocal.

Closes #471.  References #475.
Merge branch 'master' of https://github.com/tpope/vim-fugitive