Using 'vimdiff' as your git mergetool
.vimrc
" vimdiff
if &diff
map <silent> <leader>1 :diffget LOCAL<CR>
map <silent> <leader>2 :diffget BASE<CR>
map <silent> <leader>3 :diffget REMOTE<CR>
map <silent> <leader>u :diffupdate<CR>
map <silent> <leader>s :wqall<CR>