Allow arbitrary command to generate commits for git-icing

I am using a script to generate commit hashes for `git cherry-menu` to use as such:

```
git cherry-menu ./my-script.sh
```

It'd be nice if I could use the same script as input to `git-icing`

Start by comparing the `git cherry-menu ./my-script.sh` entry point with how `git-icing` currently accepts input. Trace the command handling and check whether an arbitrary script can be passed through to generate commit hashes. Done means the same script works as input to `git-icing` and produces the expected commits.

Written by the indexing model from the issue text.