[FIX JENKINS-29066] Revert "Can poll without workspace"

This reverts commit ccd4efa.

src/main/java/hudson/plugins/git/GitSCM.java

@@ -518,7 +518,7 @@ private boolean requiresWorkspaceForPolling(EnvVars environment) {
         for (GitSCMExtension ext : getExtensions()) {
             if (ext.requiresWorkspaceForPolling()) return true;
         }
-        return false;
+        return getSingleBranch(environment) == null;
     }
 
     @Override
@@ -566,6 +566,8 @@ private PollingResult compareRemoteRevisionWithImpl(Job<?, ?> project, Launcher
 
         final EnvVars pollEnv = project instanceof AbstractProject ? GitUtils.getPollEnvironment((AbstractProject) project, workspace, launcher, listener, false) : lastBuild.getEnvironment(listener);
 
+        final String singleBranch = getSingleBranch(pollEnv);
+
         if (!requiresWorkspaceForPolling(pollEnv)) {
 
             final EnvVars environment = project instanceof AbstractProject ? GitUtils.getPollEnvironment((AbstractProject) project, workspace, launcher, listener, false) : new EnvVars();
@@ -654,7 +656,6 @@ private PollingResult compareRemoteRevisionWithImpl(Job<?, ?> project, Launcher
 
             listener.getLogger().println("Polling for changes in");
 
-            final String singleBranch = getSingleBranch(pollEnv);
             Collection<Revision> candidates = getBuildChooser().getCandidateRevisions(
                     true, singleBranch, git, listener, buildData, new BuildChooserContextImpl(project, null, environment));