Include commit in version information

So I'm not entirely sure if this is even possible, but it would be nice to be able to see the current commit in the version information for beets.

This would help with debugging issues when someone is on a new version (i.e. a non-final version), as we can see the exact code that they are running.

The only way I can think of doing this is somehow including it in the `setup.py` so that it is used when installing beets, but as I previously said I'm not entirely sure if this is possible.

Here's the kind of output I'm hoping for:

```
$ beet version
beets version 1.3.18 698203e
no plugins loaded
```

This wouldn't need to be included for "full" releases as there are already tags for those.

Start by reading setup.py and the entry point for the `beet version` command to understand how version information is assembled and installed. Determine how a commit identifier could be available for non-final versions without changing full release output, then verify that `beet version` reports the expected version and commit.

Written by the indexing model from the issue text.