Comment and test that we shouldn't copy for blocks

Seems important.

src/env.cpp

@@ -183,6 +183,8 @@ void var_stack_t::push(bool new_scope) {
 
     // Copy local-exported variables
     auto top_node = top.get();
+    // Only if we introduce a new shadowing scope
+    // i.e. not if it's just `begin; end` or "--no-scope-shadowing".
     if (new_scope) {
         if (!(top_node == this->global_env)) {
             for (auto& var : top_node->env) {

tests/test3.in

@@ -257,6 +257,15 @@ set -lx var wuwuwu
 __fish_test_local_export
 echo $var
 
+# Test that we don't copy local-exports to blocks.
+set -lx var foo
+begin
+    echo $var
+    set var bar
+    echo $var
+end
+echo $var # should be "bar"
+
 # clear for other shells
 set -eU __fish_test_universal_variables_variable_foo
 

tests/test3.out

@@ -23,6 +23,9 @@ __fish_test_env17=UNSHADOWED
 wuwuwu
 boo
 wuwuwu
+foo
+bar
+bar
 Testing Universal Startup
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