CLI: allow arbitrary `git log` output format

Would be useful to be able to support arbitrary `--format` strings, so that for example you could obtain a list of commit authors in order to satisfy the use case "who should I give a heads up about my feature branch?"

Start by tracing the CLI path that invokes git log and inspect how its output is currently selected or parsed. Add support for arbitrary --format strings, then verify that a format producing commit authors works for the stated feature-branch notification use case.

Written by the indexing model from the issue text.