Don't trigger job without changes to branch (#102)

Set the SCMSource Id with the GitLabProject pathWithNamespsce so that the job won't keep building when there are no  changes on the branch (for example on periodic organization folder scan).

See also: #45, #101

src/main/java/argelbargel/jenkins/plugins/gitlab_branch_source/GitLabSCMSource.java

@@ -68,7 +68,7 @@ public GitLabSCMSource(@Nonnull String projectPath, @Nonnull GitLabSCMSourceSett
     }
 
     GitLabSCMSource(GitLabProject project, GitLabSCMSourceSettings sourceSettings) {
-        super(null);
+        setId(project.getPathWithNamespace());
         this.project = project;
         this.sourceSettings = sourceSettings;
         this.hookListener = GitLabSCMWebHook.createListener(this);