MacOS – git commit editor error ‘subl -n -w’

vi is existing with a non-zero status, although without additional details about your setup, it's difficult to tell why. If you're using a lot of plugins to vim, you might try moving your .vimrc file to .vimrc.back and seeing if you can replicate. There are a few sources that discuss this problem, including at least one with a potential solution:

git config --global core.editor /usr/bin/vim

I would try removing your .vimrc first, however. It seems to cause more consternation.

The problem was indeed a faulty configuration directive:

[init]
     templatedir=~git/init

which I accidently copied from another machine, without copying the template directory as well.