Run CI/CD only on Source Code (#824)
Run CI/CD only on Source Code (#824)

* Jenkins will now skip ci/cd pipeline if source code file paths defined in the Jenkinsfile do not have changes.

* Compare diff between current commit hash and develop.

* Escaped dollar

* Using git to get commit hash rather than jenkins variables.

* Refactored the code

* Using CHANGE_TARGET instead of hardcoded develop

Using CHANGE_TARGET instead of hardcoded develop when verifying source code changes to make this work on PRs that do not target develop.

* Changed source code paths

stan-dev/cmdstan,stan-dev/cmdstan,stan-dev/cmdstan,stan-dev/cmdstan