Fix the R manifest sub-dependencies
Fix the R manifest sub-dependencies

- Refactor commit message initialization
- Accomodate stack sub-dependency differences in commit message

[#166826002]

Co-authored-by: Guillermo Kardolus <f995e5708409e0468f92322e706c77752cc6c0d4@pivotal.io>

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci