Artifactless propagation performance optimization

[davidferlay]

Context

• Remove the need of comparison artifact in order to make propagation self-sufficient #60 has introduced a lot of changes and a new way to propagate versions across resources
• Now that it works as expected, let's optimize processing time as discussed

Todo

• Iterate timeline from latest to oldest to avoid processing same resource more than one time
• Reduce git history iteration
• Determine only used resources and look their versions
• Determine only used variables and look their versions