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.
We'll keep the Homebrew installer pointing to Tigerbrew for now but as
Homebrew/brew technically has no reason to not work on 10.5 let's remove
this check.
