Diff file tree tweaks (#21446)

- Left-align the diff stat line again like previously.
- Default the file tree to collapsed view, which means the tree will
rendered initially collapsed and it may "pop in" via JS if enabled. I
think this is more desirable than having the empty space for the tree
"pop out" like it currently does.
- Mute the icon, removing color unless hovered.
- Increase icon size and vertically center it.

Before:
<img width="1271" alt="image"
src="https://user-images.githubusercontent.com/115237/195666451-55771595-0525-42b8-be1b-d03cc1cb2961.png">


After:
<img width="1280" alt="image"
src="https://user-images.githubusercontent.com/115237/195666385-c91fd0de-6dcc-4d9c-89ff-7581828fcf14.png">

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/less/_repository.less

@@ -3099,11 +3099,11 @@ td.blob-excerpt {
 
 @media @mediaMdAndDown {
   #diff-file-tree {
-    display: none;
+    display: none !important;
   }
 
   .diff-toggle-file-tree-button {
-    display: none;
+    display: none !important;
   }
 }