Use validation message enum when creating messages

Otherwise "warning:" is printed after "commit xyz:" when pushing and
then it isn't colorized by git.

Change-Id: I6c492fb081fc7697aa61125a28359a126051be0f

src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java b/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
index 97c87ea..a7c0cad 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/validators/CommitMessageLengthValidation.java
@@ -65,10 +65,10 @@
   private void onLineTooLong(List<CommitValidationMessage> messagesList, final String message)
       throws CommitValidationException {
     if (rejectTooLong) {
-      messagesList.add(new CommitValidationMessage(message, true));
+      messagesList.add(new CommitValidationMessage(message, CommitValidationMessage.Type.ERROR));
       throw new CommitValidationException("Commit length validation failed", messagesList);
     }
-    messagesList.add(new CommitValidationMessage("warning: " + message, false));
+    messagesList.add(new CommitValidationMessage(message, CommitValidationMessage.Type.WARNING));
   }
 
   @Override