post replacements: don't delete files still in use.

Bug: if a user replaces a post with another image, then replaces the
post back to the original image, then the deletion job for the original
image will still run. The will delete the original file, but that file
is now in use again and should not be deleted.

test/unit/post_replacement_test.rb

@@ -170,5 +170,25 @@ class PostReplacementTest < ActiveSupport::TestCase
         assert_equal([{"file"=>"000000.jpg", "delay"=>125}, {"file"=>"000001.jpg", "delay"=>125}], @post.pixiv_ugoira_frame_data.data)
       end
     end
+
+    context "a post that is replaced to another file then replaced back to the original file" do
+      should "not delete the original files" do
+        @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+        @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
+        @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+
+        assert(File.exists?(@post.file_path))
+        assert(File.exists?(@post.preview_file_path))
+        assert(File.exists?(@post.large_file_path))
+
+        Timecop.travel(Time.now + PostReplacement::DELETION_GRACE_PERIOD + 1.day) do
+          Delayed::Worker.new.work_off
+        end
+
+        assert(File.exists?(@post.file_path))
+        assert(File.exists?(@post.preview_file_path))
+        assert(File.exists?(@post.large_file_path))
+      end
+    end
   end
 end