Run post-rewrite hook after rewriting commit message

[jwatzman]

This is something that libgit2 (deliberately) does not do and leaves up to application authors. It's useful for us to do, to, e.g., inform things like git-branchless (https://github.com/arxanas/git-branchless) what has happened when we rewrite commits. And it's easy enough, there is a crate specifically to do this for us.

We may want to do something similar for rebase_commits too, but this was the one that was causing me grief for now.

Test Plan: use spr amend on a repo alongside the aforementioned git-branchless. Its post-rewrite hook is invoked and its data structures are updated correctly.

[jwatzman]

If you were invoking the post-commit hook then spr could become quite slow because it can create a couple commits each time you use it.

Right -- despite creating commits under the hood, what we're doing here is morally a rewrite. (Not that git itself distinguishes between those anyway!)

It's a bit strange that spr would invoke post-rewrite but not other hooks, but I think that's ok.

We can always add the others as/if we need!

Other thing to consider is whether to have a flag in the config that decides whether this hook should run or not. I think that that might actually be a good idea. Mostly because I expect most people will not care about hooks for commits that spr creates. However, I don't feel strongly about this.

Running the hooks seems like a reasonable default behaviour, but if it's a problem, can add the options in a future PR.