Adjust sticky pr header to cover background (#23956) (#23999)

Backport #23956 by @silverwind

Very minor CSS tweak: Adjust sticky PR header to cover the box-shadow of
selected files.

Before:
<img width="1250" alt="Screenshot 2023-04-06 at 22 54 59"
src="https://user-images.githubusercontent.com/115237/230492218-4d71da48-a362-4c52-a7f7-01daf4ffa458.png">

After:
<img width="1255" alt="Screenshot 2023-04-06 at 22 54 46"
src="https://user-images.githubusercontent.com/115237/230492227-c7142210-e535-4da8-b610-37d33dcbb549.png">

Co-authored-by: silverwind <[email protected]>
Co-authored-by: Lunny Xiao <[email protected]>

Author: 3 people

web_src/css/repository.css

@@ -1631,10 +1631,10 @@
   top: 0;
   z-index: 8;
   border-bottom: none;
-  padding-left: 2px;
-  padding-right: 2px;
-  margin-left: -1px;
-  margin-right: -1px;
+  padding-left: 5px;
+  padding-right: 5px;
+  margin-left: -4px;
+  margin-right: -4px;
 }
 
 .repository .diff-detail-box > div::after {