Extract a ListRenderer struct

I'm doing this not so much because it's a great abstraction, but just because it
will make it much easier to write tests for it.

pkg/gui/context/remote_branches_context.go

@@ -43,9 +43,11 @@ func NewRemoteBranchesContext(
 				Focusable:  true,
 				Transient:  true,
 			})),
-			list:              viewModel,
-			getDisplayStrings: getDisplayStrings,
-			c:                 c,
+			ListRenderer: ListRenderer{
+				list:              viewModel,
+				getDisplayStrings: getDisplayStrings,
+			},
+			c: c,
 		},
 	}
 }