Hide file borders on sticky diff box (#22217)

Before: (1px border left and right when scrolled to a file)
<img width="1143" alt="Screenshot 2022-12-22 at 15 37 54"
src="https://user-images.githubusercontent.com/115237/209158082-c1a413b1-45b7-46b7-a71c-8e5a06324f43.png">

After: (no border)
<img width="1149" alt="Screenshot 2022-12-22 at 15 39 01"
src="https://user-images.githubusercontent.com/115237/209158086-9b00641f-2f41-4de1-9c08-22230c8a966a.png">

Layout in the box does not shift with the changes.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/_repository.less

@@ -1595,6 +1595,8 @@
       border-bottom: 1px solid var(--color-secondary);
       padding-left: 2px;
       padding-right: 2px;
+      margin-left: -1px;
+      margin-right: -1px;
     }
 
     > div::after {