For tagged commits produces the output:
- `1.0.1`
For untagged commits with a dirty tree produces the output:
- `1.0.1-19-g23efbc5-dirty`
Performance:
```
git describe --tags --dirty 2> /dev/null
0.07s user 0.01s system 96% cpu 0.086 total
```
This means we can tag any commit without needing to manually remember
to bump the revision every time.
f0e9292acd