git: Alias changes to log HEAD...FETCH_HEAD

I would have liked to call the alias `whatchanged` but am not able to as
git brings a command named liked that (for backwards compatibility) and
alias cannot override builtins.

.config/git/config

@@ -8,6 +8,7 @@
 	autosquash = autofixup
 
 	branches = track
+	changes = flog HEAD...FETCH_HEAD
 	checkout-worktree = "!\"${XDG_CONFIG_HOME:-$HOME/.config}/zsh/autoload/git/git-checkout-worktree\""
 	filter-repo = "!\"${XDG_CONFIG_HOME:-$HOME/.config}/git/git-filter-repo/git-filter-repo\""
 	fixes = log -1 --pretty=fixes