Merge branch 'stable-2.12'

* stable-2.12:
  Update 2.12.4 release notes
  Fix RebaseIfNecessary strategy introducing duplicate commit
  Handle an NPE in ToolsCatalog.java
  Confirm email: Return proper error when email is used by other account
  RestApiServlet: Allow HEAD requests

Change-Id: I15414c40f1136236708cca17128bf60fa020c160

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/HttpResponse.java

@@ -51,6 +51,16 @@ public class HttpResponse {
     return response.getStatusLine().getStatusCode();
   }
 
+  public String getContentType() {
+    return response.getFirstHeader("X-FYI-Content-Type").getValue();
+  }
+
+  public boolean hasContent() {
+    Preconditions.checkNotNull(response,
+        "Response is not initialized.");
+    return response.getEntity() != null;
+  }
+
   public String getEntityContent() throws IOException {
     Preconditions.checkNotNull(response,
         "Response is not initialized.");