Add info about branches and stash to robbyrussel fish_prompt

I always forget whether there are stashed changes or branches I haven't pushed yet. After accidentally blowing away a few important stashed changes, I decided that it would be a good idea to have that info (num of local branches, besides master and a stash flag) in the prompt.

With only the "master" branch and stashed changes it looks the same as before:

```
➜  fish-shell git:(master)
```

Here's how it looks with one local branch, and a stashed change:

```
➜  fish-shell git:(master 1B S)
```

Hope others find it useful.