apply change: Fix adding component with non default simple attributes.

When adding a component with simple attributes, these attributes might
have default values that aren't `nil`. Previously, when such a thing
occurred it would result in a `MergeError`, because the removed change
was assumed to be `nil` since the component was just added, but in
practice it would be set to the default value.
The solution is to set the removed change value to the default value.

lib/kintsugi/apply_change_to_project.rb

@@ -676,7 +676,10 @@ def add_attributes_to_component(component, change, change_path, ignore_keys: [])
 
         attribute_name = attribute_name_from_change_name(change_name)
         if simple_attribute?(component, attribute_name)
-          apply_change_to_simple_attribute(component, attribute_name, {added: change_value})
+          apply_change_to_simple_attribute(component, attribute_name, {
+            added: change_value,
+            removed: simple_attribute_default_value(component, attribute_name)
+          })
           next
         end
 
@@ -694,6 +697,12 @@ def add_attributes_to_component(component, change, change_path, ignore_keys: [])
       end
     end
 
+    def simple_attribute_default_value(component, attribute_name)
+      component.simple_attributes.find do |attribute|
+        attribute.name == attribute_name
+      end.default_value
+    end
+
     def find_file(project, file_reference_change, file_filter: ->(_) { true })
       file_references = project.files.select do |file_reference|
         file_reference.path == file_reference_change["path"] && file_filter.call(file_reference)

spec/kintsugi_apply_change_to_project_spec.rb

@@ -1062,6 +1062,20 @@
       expect(base_project).to be_equivalent_to_project(theirs_project)
     end
 
+    it "adds build phase with a simple attribute value that has non nil default" do
+      theirs_project = create_copy_of_project(base_project.path, "theirs")
+
+      theirs_project.targets[0].new_shell_script_build_phase("bar")
+      theirs_project.targets[0].build_phases.last.shell_script = "Other value"
+
+      changes_to_apply = get_diff(theirs_project, base_project)
+
+      described_class.apply_change_to_project(base_project, changes_to_apply)
+      base_project.save
+
+      expect(base_project).to be_equivalent_to_project(theirs_project)
+    end
+
     it "removes build phase" do
       base_project.targets[0].new_shell_script_build_phase("bar")