How do I make Git use the editor of my choice for commits?

Question

I would prefer to write my commit messages in Vim, but it is opening them in Emacs.

How do I configure Git to always use Vim? Note that I want to do this globally,

Answer 1

On macOS Big Sur (11.0) beta for TextMate: none of the environment variable options worked. (Set all three: GIT_EDITOR, VISUAL, and EDITOR.)

Finally set the global core.editor in git, and that worked:
git config --global core.editor "~/bin/mate -w"