CommitMergeStatus: Make message member final

Otherwise error prone complains about it [1]:

  enums should be immutable, and cannot have non-final fields

[1] http://errorprone.info/bugpattern/ImmutableEnumChecker

Bug: Issue 5823
Change-Id: I2c0570afd7d53b76305e7407d57cea741873de54

gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/CommitMergeStatus.java

@@ -62,7 +62,7 @@ public enum CommitMergeStatus {
           + "\n"
           + "Please rebase the change locally and upload again for review.");
 
-  private String message;
+  private final String message;
 
   CommitMergeStatus(String message) {
     this.message = message;