Add this to your
.vimrc to automatically start in Insert mode when committing in Git. This will also enable spellchecking.
if has('autocmd') if has('spell') au BufNewFile,BufRead COMMIT_EDITMSG setlocal spell endif au BufNewFile,BufRead COMMIT_EDITMSG call feedkeys('ggi', 't') endif
Of course, you'll need to make sure you're using Vim as your editor when committing.