[JENKINS-50777] Exporting AbstractGitSCMSource.SCMRevisionImpl.hash

[jglick]

JENKINS-50777 - allow commit hash to be retrieved from multibranch build metadata via REST

Useful in conjunction with jenkinsci/scm-api-plugin#52.

Checklist

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• I have read the CONTRIBUTING doc
• I have referenced the Jira issue related to my changes in one or more commit messages
• I have added tests that verify my changes
• Unit tests pass locally with my changes
• I have added documentation as necessary
• No Javadoc warnings were introduced with my changes
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• Any dependent changes have been merged and published in upstream modules (like git-client-plugin)

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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