HashtagsInput: Add a constructor allowing to initialize remove member

The class already has a similar constructor, but it only allows to
initialize the 'add' member. This means that if one wants to create
an instance with both 'add' and 'remove', it is still necessary to
instantiate it and then explicitly set 'remove'.

Change-Id: Iacb8ca5b708d848dc39c1d48803f1ad312a7ec19

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/HashtagsInput.java

@@ -26,4 +26,9 @@ public class HashtagsInput {
   public HashtagsInput(Set<String> add) {
     this.add = add;
   }
+
+  public HashtagsInput(Set<String> add, Set<String> remove) {
+    this(add);
+    this.remove = remove;
+  }
 }