GHRespository.getTemplateRepository() should call populate() if null

[kkroner8451]

Describe the bug
A clear and concise description of what the bug is.

When calling the GitHub API to get a list of repositories the template_repository field is not included in the response. Thus if using this library to get a list of repositories (i.e. GHOrganization.listRepositories()) that field will always be null when mapped onto the GHRepository object constructed as introduced in PR#1817. Under a different proposal I saw PR#1579 it had a check

 if this.template_repository == null 
  populate()

This has its own performance concerns as there is no flag that populate() isn't called more than once which would happen if this was not a child of a template repository and the getTemplateRepository() method was called multiple times. All works as expected when getting a single repository via GitHub.getRepository() as the GitHub API includes the template_repository field in that case.

To Reproduce
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4. See error

GHRepositories[] repos = gitHub.getOrganization("SAMPLE").listRepositories(10);
for (GHRepository repo : repos){
  if (repo.getTemplateRepository() != null) {
   // BUG: This will never hit as underlying template_repository field isn't ever populated when using list vs single get methods
  doSomethingWithTemplate();
  }
}

Expected behavior
A clear and concise description of what you expected to happen.

I expect the GHRepository.getTemplateRepository() to return the value assuming the repo is a child of a template regardless of if it was originally constructed by a method to get the single repo (working) versus getting a list of repositories (not working) and believe making the delayed load populate() call as seen in other places is an established behavior that would be expected.

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• Version [e.g. 22]

Additional context
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