Change how we separate toplevel and global scopes

Instead of introducing a new local scope at the point of `set`, merely
push a new local scope at the end of env_init(). This means we have a
single toplevel local scope across the lifetime of the fish process,
which means that

    set -l foo bar
    echo $foo

behaves as expected, without modifying the global environment.

env.cpp

@@ -593,6 +593,14 @@ void env_init(const struct config_paths_t *paths /* or NULL */)
 
     /* Set fish_bind_mode to "default" */
     env_set(FISH_BIND_MODE_VAR, DEFAULT_BIND_MODE, ENV_GLOBAL);
+
+    /*
+      Now that the global scope is fully initialized, add a toplevel local
+      scope. This same local scope will persist throughout the lifetime of the
+      fish process, and it will ensure that `set -l` commands run at the
+      command-line don't affect the global scope.
+    */
+    env_push(false);
 }
 
 /**
@@ -1195,16 +1203,6 @@ void env_pop()
     }
 }
 
-bool env_ensure_local_scope()
-{
-    if (top == global_env)
-    {
-        env_push(false);
-        return true;
-    }
-    return false;
-}
-
 /**
    Function used with to insert keys of one table into a set::set<wcstring>
 */