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`.

README.md

@@ -8,8 +8,10 @@ rather than remembering the plumbing term of a specific command.
 * `abandon` abandons all local changes leave a clean repository.
 * `unstage` unstage staged changes, shorthand for `git reset -q HEAD --`.
 * `amend` amends the last commit, shorthand for `git commit --amend`.
-* `fixup` prepare to fixup a commit, shorthand for `git commit --fixup`.
-* `squash` prepare to squash a commit, shorthand for `git commit --squash`.
+* `fixup` prepare a fixup commit for use with
+  `git rebase --interactive --autosquash`, shorthand for `git commit --fixup`.
+* `squash` prepare a squash commit for use with
+  `git rebase --interactive --autosquash`, shorthand for `git commit --squash`.
 * `prepare` prepare a branch for merging into another, shorthand for
   `git rebase --interactive --fork-point`.
 * `list` list local branches, a shorthand for `git branch`.