* Make commit of new version optional in `artifactSetVersion` step
You might not want to add a new commit for each version, when versions
are automatically created. This commit makes this feature optional, but
enabled by default to maintain API compatibility.
* Set `gitCommitId` if null
* Improve documentation of `commitVersion` parameter
* Allow to operate in a code checkout without `.git` directory
The characteristics of a list are
o the order of the entries is significant
o duplicates are allowed
The characteristics of a set are
o the order is not significant
o duplicates are not allowed.
When describing keys for a step the characteristics of a
set applies here, whereas the characteristics of a list does
not apply.
