agt file can not get the correct path after run tabnew and lcd

[wsdjeg]

ref: wsdjeg/SpaceVim#1045

@cohama in this issue

1. we are in ~/SpaceVim/SpaceVim, and this is a root of a git repo
2. we use tabnew ~/.SpaceVim.d/init to open the custom file this is a link to ~/Dotfiles/SpaceVim.d/init.vim. and ~/Dotfiles also is a root of git repo.
3. and we use vim-rooter to auto set root dir
4. when we use AgitFile, it cause some error, and when we use Agit, it show log of old path

maybe This plugin should handle the local path instead.