BURs: fix mass update A -> B not being allowed.

Fix mass updates of the form `mass update A -> B` not being allowed.

This was originally because after `rename` was introduced, we wanted to
prevent people from using mass updates to move tags. Now, `mass update A -> B`
adds B to all posts tagged A instead of moving A to B. So `mass update A -> B`
should no longer be disallowed.

This also makes it so that it's an error to create a mass update with a
syntax error in the search. Before searches couldn't have syntax errors,
but now with the new query parser it's possible.

app/logical/bulk_update_request_processor.rb

@@ -116,11 +116,10 @@ class BulkUpdateRequestProcessor
           end
 
         when :mass_update
-          lhs = PostQuery.new(args[0])
-          rhs = PostQuery.new(args[1])
+          query = PostQuery.new(args[0])
 
-          if lhs.is_single_tag? && rhs.is_single_tag?
-            errors.add(:base, "Can't mass update #{args[0]} -> #{args[1]} (use an alias or a rename instead for tag moves)")
+          if query.is_null_search?
+            errors.add(:base, "Can't mass update #{args[0]} -> #{args[1]} (the search `#{args[0]}` has a syntax error)")
           end
 
         when :nuke

app/logical/post_query.rb

@@ -16,6 +16,16 @@ class PostQuery
     Tag.where(name: tag_names)
   end
 
+  # True if this search would return all posts (normally because the search is the empty string).
+  def is_empty_search?
+    ast.all?
+  end
+
+  # True if this search would return nothing (normally because there was a syntax error).
+  def is_null_search?
+    ast.none?
+  end
+
   def is_single_tag?
     ast.tag?
   end

test/unit/bulk_update_request_test.rb

@@ -339,11 +339,11 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
           assert_equal("approved", @bur.reload.status)
         end
 
-        should "not allow mass update for simple A -> B moves" do
-          @bur = build(:bulk_update_request, script: "mass update bunny -> rabbit")
+        should "not allow invalid searches in mass updates" do
+          @bur = build(:bulk_update_request, script: "mass update (foo -> bar")
 
           assert_equal(false, @bur.valid?)
-          assert_equal(["Can't mass update bunny -> rabbit (use an alias or a rename instead for tag moves)"], @bur.errors.full_messages)
+          assert_equal(["Can't mass update (foo -> bar (the search `(foo` has a syntax error)"], @bur.errors.full_messages)
         end
       end