Add shortlog script to create git shortlog for each layer.

[chuanjin]

To get an overview of what changed in the layers used between two commits of our manifest, we create a shortlog for each layer and log that in a Jenkins job.

[rpannek]

Does it output things to stdout now? Perhaps it should be changed from "Source the environment and create git shortlog for each layer." to "Source the environment and print git shortlog for each layer."

[chuanjin]

Yes, it will output to stdout now. I have updated the comments. @rpannek @tobsan