Add a new command: UnsupportedCommand to communicate compatibility of JGit

[rishabhBudhouliya]

This command serves as a way to communicate to the git plugin which git features
are not supported by JGit.

• The reason for not implementing it as a GitCommand implementation is to separate
  the need of a JGitAPIImpl or CliGitAPIImpl to check if a certain additional behaviour
  of git is supported or not.
• This class can be instantiated with no need of any information, it can be decorated by a
  GitSCMExtension to fill in the features which are not supported.

How to use it in the Git Plugin?

• Step 1: Define a function within the GitSCMExtension to be implemented by extensions which are not supported by JGit.
  ex: public void determineSupportForJGit(GitSCM scm, UnsupportedCommand unsupportedCommand) { }
• Step 2: Let's take GitLFSPull as an example, implement a function called:

    public void determineSupportForJGit(GitSCM scm, UnsupportedCommand cmd) {
        List<RemoteConfig> repos = scm.getRepositories();
        cmd.lfsRemote(repos.get(0).getName()); 
}

• Step 3: Use the collected information to determine if JGit can be supported or not using the UnsupportedCommand only.

Checklist

• I have read the CONTRIBUTING doc
• 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

What types of changes does your code introduce? Put an x in the boxes that apply

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

[rishabhBudhouliya]

@MarkEWaite Many thanks for writing the test cases, I was working on the git plugin PRs and couldn't add those in time.

While the automated cases written should be more than enough to cover tests functionality wise, I believe we may have to add more test cases within the git plugin to check if the UnsupportedCommand is able to determine support after being used by various extensions.

[MarkEWaite]

@rishabhBudhouliya I saw that the message associated with the first commit in this pull request was missing the blank line after the first line. That causes the log output of command line git to be "noisy". I modified the commit message to add that blank line so that the log output would be more tidy. I also added javadoc comments for all the public methods of the UnsupportedCommand class so that we don't have javadoc warnings added to the build.

I believe this is ready to merge once the CI job passes.