Reference: Using Git from command-line mode
Set core.editor
git config --global core.editor vim
# check your config
git config --list
Using Git from command line
# use :!
# add the current file
:!git add %
:!git commit -m "commit message"
:!git push origin master
:!git diff --cached
Adding key mapping
nnoremap \ga :<C-u>!git add %<CR>