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.

builtin_set.cpp

@@ -566,17 +566,6 @@ static int builtin_set(parser_t &parser, wchar_t **argv)
     */
     scope = (local ? ENV_LOCAL : 0) | (global ? ENV_GLOBAL : 0) | (exportv ? ENV_EXPORT : 0) | (unexport ? ENV_UNEXPORT : 0) | (universal ? ENV_UNIVERSAL:0) | ENV_USER;
 
-    /*
-      If we're interacting with the local scope at all, ensure we actually have
-      one that's distinct from the global scope. If we don't have one yet,
-      create one and modify the current block to pop it.
-    */
-    if ((scope & ENV_LOCAL) && env_ensure_local_scope())
-    {
-        block_t *block = parser.current_block();
-        block->wants_pop_env = true;
-    }
-
     if (query)
     {
         /*

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>
 */

env.h

@@ -206,12 +206,6 @@ void env_push(bool new_scope);
 */
 void env_pop();
 
-/**
-  Push the variable stack if the current scope is the global scope. Returns
-  whether it pushed a scope.
-*/
-bool env_ensure_local_scope();
-
 /** Synchronizes all universal variable changes: writes everything out, reads stuff in */
 void env_universal_barrier();