Add option to delete/not produce log files if "no changes"

Relates to
- #124

For many (if not most) usecases of `datalad run` usage it would be desired to prevent committing anything if the only change is the `.duct/` logs.

Ideas:
- configurable/pluggable interfaces (multiple could be specified) to decide on either to keep or not. E.g. `--keep-if always|git-dirty|success|error`. E.g. `--keep-if git-dirty,success` so we do not even keep logs if errors out.
- could be complemented with `--run-if always|git-clean`, somewhat mimic'ing `datalad run` behavior... but no good use case yet
- on datalad level somehow "kill" that commit with only `.duct/` changes?
- ...

Start with the datalad run workflow and the related #124 discussion. Clarify whether the desired scope is configurable log retention, conditional execution, or avoiding commits containing only .duct/ logs, then define the supported keep-if behavior and how completion will be verified.

Written by the indexing model from the issue text.