CreateMergePatchSet: Implement 'author' to tweak Git commit author

This change implements the same functionality as I54b98be6 but for
CreateMergePatchSet. Setting the newly exposed 'author' field requires
the caller to have 'forgeAuthor' permission.

Change-Id: Ia434ed0ed11489296d8c2aa9f8208faac293b563

java/com/google/gerrit/extensions/common/MergePatchSetInput.java

@@ -14,9 +14,12 @@
 
 package com.google.gerrit.extensions.common;
 
+import com.google.gerrit.extensions.api.accounts.AccountInput;
+
 public class MergePatchSetInput {
   public String subject;
   public boolean inheritParent;
   public String baseChange;
   public MergeInput merge;
+  public AccountInput author;
 }