Relax URL matching for GitHub Webhook triggered builds.

Prior to this patch, the code would only match URLs that ended with '.git', which
is not required by GitHub. This patch allows projects to use URLs with or without
the '.git' suffix.

Addresses the URL matching parts of JENKINS-10405, but not the security/authorization
parts.
Refactor GitHubRepositoryName tests to simplify code.
Add a comment documenting the need for pattern duplication.
[JENKINS-10405] Merge branch 'pull-24'

This pull request relaxes the rule to allow the github URL that does not
end with '.git'