Fix file list jump caused by `toggle-files-button`

Fixes #2160

Without this change, the DOM would be:

```html
<include-fragment src="/sindresorhus/refined-github/tree-commit/3a55f541b11e78eff2484eb409f704e0eae9b32e"
	class="commit-tease commit-loader">
	<div class="blank loader-loading">
		<img alt="" class="loader" src="https://github.githubassets.com/images/spinners/octocat-spinner-32-EAF2F5.gif"
			width="16" height="16">
		Fetching latest commit…
	</div>
	<div class="loader-error">
		Cannot retrieve the latest commit at this time.
	</div>
	<button class="btn-octicon rgh-toggle-files" aria-label="Toggle files section" aria-expanded="true"><svg
			aria-hidden="true" class="octicon octicon-chevron-down" width="10" height="16">
			<path fill-rule="evenodd" d="M10 10l-1.5 1.5L5 7.75 1.5 11.5 0 10l5-5z"></path>
		</svg></button>
</include-fragment>
```

1 files changed, 2 insertions, 1 deletions

diff --git a/source/features/toggle-files-button.tsx b/source/features/toggle-files-button.tsx
index e1a04c3c..d36fca4a 100644
--- a/source/features/toggle-files-button.tsx
+++ b/source/features/toggle-files-button.tsx
@@ -7,7 +7,8 @@ import * as icons from '../libs/icons';
 import observeEl from '../libs/simplified-element-observer';
 
 function addButton(): void {
-	const filesHeader = select('.commit-tease');
+	// `div` excludes `include-fragment`, which means the list is still loading. #2160
+	const filesHeader = select('div.commit-tease');
 	if (!filesHeader || select.exists('.rgh-toggle-files')) {
 		return;
 	}