"Copy to clipboard" button on diff file header is ambiguous

If you didn't already know what this button did, would you think it copies the filename to the clipboard, or the contents of the file? I knew we added this feature and even I had to think about it for a second. Especially because on a Snippet page, the button copies the contents.

Design

The button should be replaced directly after the title like so:

Let's update the tooltip as well With:

"Copy filename to clipboard"

cc @awhildy