Print buildpack repo info to standard output (under compile script)

This can only be printed under the compile script as the detect and
release scripts deliver payloads to standard output.

If there is a git repository, prints the following:

    Java Buildpack source: <remote url>#<head commit sha>

If there is no git repository, prints the following:

    Java Buildpack source: system

[#62829398]
Merge 62829398-display-git-commit to master

[Completes #62829398]