[JENKINS-57660] Addition of a doCheck validation method for invalid refSpec

[rishabhBudhouliya]

JENKINS-57660

While adding a Git source in the Source Code Management, adding an invalid refSpec produces a confusing/complex stack trace upon applying/saving the configuration changes.

Upon discussion on this issue, a good way to inform the user about the invalid refSpec is to check the correctness of the values entered by the user on the fly (Done on the server-side). Hence, a doCheckRefspec method has been added to create a form validation logic.

Here is a screenshot of the UI result of the new method:

Note: The message has been changed to "Specification is invalid"

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
• I have interactively tested my changes
• Any dependent changes have been merged and published in upstream modules (like git-client-plugin)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

[MarkEWaite]

Thanks so much for this pull request! I've made comments on areas where I would like changes (add tests of more success cases and at least one failure case) and on areas where I need to investigate further (confirm that getAllRemoteConfigs does not connect to the remote git repository)

[MarkEWaite]

Thanks again for the submission. Minor updates on my comments.

I clicked the "is documented" checkbox in the template because I don't think any change is needed to the README for this. You're providing user input validation that should have been provided before.

[MarkEWaite]

Thanks for the additional tests. Very nice!

One open question on a possible unintentional behavior change from the current releases to this change.

[rishabhBudhouliya]

@MarkEWaite I have added another test-case as requested, you can check it once and merge the PR.
Thank you for taking out the time to review it!