[JENKINS-73677] Reapply: Decorate GitClient after adding credentials

[Dohbedoh]

(second attempt of #1649)

JENKINS-73677: Credentials are added to GitClient after the client is decorated by extensions.. A plugin cannot for example override the credentials passed in via a GitSCMExtension (Noticed as part of jenkinsci/bitbucket-branch-source-plugin#867)

• Previous attempt to apply this caused problems JENKINS-73797 for Bitbucket Branch Source user that use the SSH Credentials trait.
• The PR [JENKINS-73471] Git scm.userRemoteConfigs.credentialsId configured by Bitbucket is null bitbucket-branch-source-plugin#899 has been merged in Bitbucket branch source to prevent this problem for happening again (but unreleased at the moment released as 906.vedf430cb_4481).
• Now proposing to apply the change again in the Git plugin to be able to move forward.
• Will use the incrementals of this PR to rebase and test [JENKINS-73471] Restore passing credentialsId to the GitSCM bitbucket-branch-source-plugin#867

Testing done

Tested simple scenario: configured Pipeline from SCM with GitSCM with provided Credentials.
Tested other scenario: configured Multibranch SCM with Bitbucket Branch Source.
Tested other scenario: configured Multibranch SCM with Bitbucket Branch Source with SSH Credentials trait.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[nfalco79]

bitbucket-branch-source-plugin has been released with workaround that make SSH works with this patch

[Dohbedoh]

@MarkEWaite Any chance to get this merged ? Bitbucket Branch Source should be ready now. cc @nfalco79

[nfalco79]

Yes please. Merging this fix, enable us to fix the authentication issues in Bitbucket plugin

[MarkEWaite]

@MarkEWaite Any chance to get this merged ? Bitbucket Branch Source should be ready now. cc @nfalco79

I'm leaving town until next Monday 2 Dec 2024. I'll plan to test it and release it the week of 2 Dec 2024.