Add aliases for working with autosquash rebases

* Rename the `squash` alias to `prepare` to repurpose it.
* Change `squash` alias to `git commit --squash=<commit>`.
* Add `fixup` alias to `git commit --fixup=<commit>`.
* Set the `rebase.autoSquash` config option to `true`.

_git-changes

@@ -1,7 +1,6 @@
 #compdef git-changes
 #description Compare changes on this branch with another.
 
-_arguments \
-  '1: :__git_remote_branch_names_noprefix'
+_arguments '1: :__git_remote_branch_names_noprefix'
 
 # vim: ft=zsh