Improve branch context info & layout

The goal with this is to improve the the branch context info that is
displayed when we run the gps ls command.

This involved rewording things to make things clearer in Git terms,
removing what seem to be needless words, and adding the concept of
showing how many patchs are in the patch stack with [ahead X].

[changelog]
added: [ahead X] to ls cmd branch context information
changed: ls cmd branch context info to be clearer

<!-- ps-id: 6030f154-9b60-415a-ab6f-2889dcd4c233 -->

src/ps/public/list.rs

@@ -55,8 +55,10 @@ pub fn list(color: bool) -> Result<(), ListError> {
             .unwrap();
 
     println!(
-        "Stack {} following {}\n",
-        &cur_patch_stack_branch_name, &cur_patch_stack_branch_upstream_name
+        "{} tracking {} [ahead {}]",
+        &cur_patch_stack_branch_name,
+        &cur_patch_stack_branch_upstream_name,
+        list_of_patches.len()
     );
 
     let list_of_patches_iter: Box<dyn Iterator<Item = _>> = if config.list.reverse_order {