git: Improve co-author button (#29742)

This PR changes the tooltip label to say "Remove" when you have the
button toggled on and collaborators in the list.

Release Notes:

- N/A

Co-authored-by: Joseph T. Lyons <josephtlyons@gmail.com>

crates/icons/src/icons.rs

@@ -238,6 +238,7 @@ pub enum IconName {
     Undo,
     Unpin,
     Update,
+    UserCheck,
     UserGroup,
     UserRoundPen,
     Visible,