Another fish var performance improvement

Make setting fish vars more efficient by avoiding creating a
wcstring_list_t for the case where we're setting one value. For the case
where we're passing a list of values swap it with the list in the var
rather than copying it. This makes the benchmark in #4200 approximately
6% faster.

src/function.cpp

@@ -345,6 +345,7 @@ void function_prepare_environment(const wcstring &name, const wchar_t *const *ar
         // It should be impossible for the var to be missing since we're inheriting it from an outer
         // scope. So we now die horribly if it is missing.
         assert(!it->second.missing());
-        env_set(it->first, ENV_LOCAL | ENV_USER, it->second.as_const_list());
+        wcstring_list_t vals = it->second.as_const_list();  // we need a copy
+        env_set(it->first, ENV_LOCAL | ENV_USER, vals);     // because this mutates the list
     }
 }