Add an "opt-out" global switch to retain second fetch

[rishabhBudhouliya]

PR-904 Fix redundant fetch

With the latest fix on redundant fetch issue [JENKINS-49757], the
second fetch in the checkout step is avoided for cases where the
second fetch is not adding any new repository information to the
existing git repository.

Enabling this switch will allow the user to preserve the second
fetch and remedy any compatibility issues.

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. This is simply a reminder of what we are going to look for before merging your code. If a checkbox or line does not apply to this pull request, delete it. We prefer all checkboxes to be checked before a pull request is merged

• 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
• I have interactively tested my changes

Types of changes

• New feature (non-breaking change which adds functionality)

[rishabhBudhouliya]

This branch was originally created to add performance improvement global switch. This switch has been used to add "opt out" feature for retaining second fetch during the checkout step.

[rishabhBudhouliya]

This has been changed to allowSecondFetch in the subsequent commits.

[MarkEWaite]

The asymmetry between getter and setter name is likely to disrupt configuration as code and JobDSL.

Could you make them follow the usual naming convention?

private boolean allowSecondFetch;
public boolean isAllowSecondFetch() {}
public void setAllowSecondFetch(boolean allowSecondFetch) {}

Interactive testing seems to show that enabling 'Allow second fetch' is not being preserved for later use in the "Manage Jenkins" page.

The following steps give unexpected results:

1. Open the "Manage Jenkins" page and find the "Git plugin" section of the page
2. Enable "Allow second fetch" by clicking the checkbox
3. Save the configuration
4. Run a job and confirm the second fetch is visible
5. Return to the "Manage Jenkins" page and find the "Git plugin" section of the page
6. Unexpectedly, the "Allow second fetch" checkbox is no longer checked

If you'd like to experiment with modifying the values without using the UI controls, you can use the "Script Console" that is available from "Manage Jenkins" with the following code:

def inst = Jenkins.getInstance()
def desc = inst.getDescriptor("hudson.plugins.git.GitSCM")
println(desc.getGlobalConfigName())
println(desc.getGlobalConfigEmail())
println(desc.isRedundantFetchAllowed())

desc.setGlobalConfigName("Mark Waite")
desc.setAllowSecondFetch(false)

println(desc.getGlobalConfigName())
println(desc.getGlobalConfigEmail())
println(desc.isRedundantFetchAllowed())

[rishabhBudhouliya]

@MarkEWaite I was also interactively testing this change and I am also facing this issue. The checkbox option is not being persisted across multiple build jobs.

I'll debug the instance for RCA.

[rishabhBudhouliya]

@MarkEWaite I have done two things:

• Interactively tested the checkbox, now the selection is persisting in my local instance.
• Removed redundant and replaced it with second.