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 Ubuntu and also Debian (thanks @MichielB) changing the default editor is also possible by running:

sudo update-alternatives --config editor

Which will prompt the following:

There are 4 choices for the alternative editor (providing /usr/bin/editor).

  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /bin/nano            40        auto mode
  1            /bin/ed             -100       manual mode
  2            /bin/nano            40        manual mode
* 3            /usr/bin/vim.basic   30        manual mode
  4            /usr/bin/vim.tiny    10        manual mode

Press enter to keep the current choice[*], or type selection number: