Only create start/end commits if changes happened

git commit by default will have exit code 1 if there is nothing
to commit - after all, why create a commit if there is no change.

There is an option '--allow-empty' to git commit, but this will
create an empty commit for each and every etckeeper run. This would
complicate our commit history, so instead only commit if there are changes
reported by git status.

Using 'onlyif' here also means that we don't have no-op state changes appearing
to happen.