Fix #4969: Tag changes made by replacements wipe out edits done at the same time.

Lock the post during replacement to ensure we have the latest version of
the tags and to ensure nobody else can modify the post until after the
replacement is finished.

test/functional/post_replacements_controller_test.rb

@@ -6,13 +6,14 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
       context "replacing a post from a source url" do
         should "replace the post" do
           assert_difference("PostReplacement.count") do
-            @post = create(:post)
+            @post = create(:post, tag_string: "image_sample")
 
             post_auth post_replacements_path, create(:moderator_user), params: {
               format: :json,
               post_id: @post.id,
               post_replacement: {
                 replacement_url: "https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg",
+                tags: "replaced -image_sample"
               }
             }
 
@@ -31,6 +32,7 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
           assert_equal(@post.file_size, @replacement.old_file_size)
           assert_equal(@post.file_ext, @replacement.old_file_ext)
           assert_equal(@post.md5, @replacement.old_md5)
+          assert_equal(@post.tag_string, "image_sample")
 
           @post.reload
           assert_equal("d34e4cf0a437a5d65f8e82b7bcd02606", @post.md5)
@@ -40,6 +42,7 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
           assert_equal(650, @post.image_height)
           assert_equal(127_238, @post.file_size)
           assert_equal("jpg", @post.file_ext)
+          assert_equal("replaced", @post.tag_string)
         end
       end