[JENKINS-64578] Support multiple SCM repositories per job

[uhafner]

Some jobs are using multiple SCM repositories: one for the code, one for the pipeline library, etc. We need a way to select the correct repository for all features of the forensics API. See also JENKINS-64578.

• Add a test case that exposes the problem for the commit decorator
• The commit recorder correctly scans both repositories
• The commit decorator works only for the second one and points to the wrong repository
• The reference build will also only use the first one
• The miner does only scan the first one
• The blamer does only work on the first one
• Select existing miner results with the filter