Adds a new command `gh auth setup-git [<hostname>]` that sets up git to
use the GitHub CLI as a credential helper.
The gist is that it runs these two git commands for each hostname the
user is authenticated with.
```
git config --global --replace-all 'credential.https://github.com.helper' ''
git config --global --add 'credential.https://github.com.helper' '!gh auth git-credential'
```
If a hostname flag is given, it'll setup GH CLI as a credential helper
for only that hostname.
If the user is not authenticated with any git hostnames, or the user is
not authenticated with the hostname given as a flag, it'll print an
error.
Co-authored-by: Mislav Marohnić <mislav@github.com>
