Change length parameter of getDisplayStrings to endIdx

It's more natural to work with this way, as we will see later in this branch.

pkg/gui/context/remote_branches_context.go

@@ -27,7 +27,7 @@ func NewRemoteBranchesContext(
 		},
 	)
 
-	getDisplayStrings := func(startIdx int, length int) [][]string {
+	getDisplayStrings := func(_ int, _ int) [][]string {
 		return presentation.GetRemoteBranchListDisplayStrings(viewModel.GetItems(), c.Modes().Diffing.Ref)
 	}