Add mnemonicPrefix to git config

This changes the labelling of git diff stats from the ambiguous 'a' and 'b' to
things like 'i'(ndex), 'w'(orktree) and 'c'(ommit), depending on the context.
See the `git-config` manpage for more information.

setup.sh

@@ -43,8 +43,9 @@ echo "We touched all the known hosts."
 
 # Git config
 git config --global color.ui true
-git config --global core.editor "vim"
+git config --global core.editor vim
 git config --global core.excludesfiles ~/dotfiles/git/globalignore
+git config --global diff.mnemonicPrefix true
 echo "Git config done."
 
 # Initialise and clone any submodules