[JENKINS-50394] Add ability for JenkinsRule tests to insert custom GitClient implementation

[rsandell]

As what is returned from Git.getClient

JENKINS-50394 - When new commits are pushed to the repository while GitSCMSource is discovering branches then MissingObjectException

To be able to verify a fix for JENKINS-50394 I needed some way of detecting various phases in the descovery process of AbstractGitSCMSource, an easy way would be if I can use a custom "mocked" version of GitClient during the test that injects some more refs in the middle of the fetch/discoverBranches sequence.

Checklist

• 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
• No spotbugs warnings were introduced with my changes

Types of changes

• Infrastructure change (non-breaking change which updates dependencies or improves infrastructure)
• New feature (non-breaking change which adds functionality)

Further comments

Added a guard so that this only activates during JenkinsRule tests so that normal production code won't be affected of able to be otherwise "persuaded".

Upstream to jenkinsci/git-plugin#702

[rsandell]

@MarkEWaite @stephenc @bitwiseman @dwnusbaum @abayer @kshultzCB @car-roll

[MarkEWaite]

Looks good to me. Won't merge it until the CI server passes tests.