Add fish_new_pager variable to enable new pager on request, disabled by default.

parser.cpp

@@ -3122,3 +3122,16 @@ bool parser_use_ast(void)
         return from_string<bool>(var);
     }
 }
+
+bool pager_use_inline(void)
+{
+    env_var_t var = env_get_string(L"fish_new_pager");
+    if (var.missing_or_empty())
+    {
+        return 0;
+    }
+    else
+    {
+        return from_string<bool>(var);
+    }
+}

parser.h

@@ -547,6 +547,7 @@ public:
 
 /* Temporary */
 bool parser_use_ast(void);
+bool pager_use_inline(void);
 
 
 #endif

reader.cpp

@@ -1876,8 +1876,9 @@ static bool handle_completions(const std::vector<completion_t> &comp)
             parse_util_get_parameter_info(data->command_line, data->buff_pos, &quote, NULL, NULL);
             bool is_quoted = (quote != L'\0');
             
-            if (1)
+            if (pager_use_inline())
             {
+                /* Inline pager */
                 data->pager.set_prefix(prefix);
                 data->pager.set_completions(surviving_completions);
                 
@@ -1886,7 +1887,7 @@ static bool handle_completions(const std::vector<completion_t> &comp)
             }
             else
             {
-                /* Clear the autosuggestion from the old commandline before abandoning it (see #561) */
+                /* Classic pager. Clear the autosuggestion from the old commandline before abandoning it (see #561) */
                 if (! data->autosuggestion.empty())
                     reader_repaint_without_autosuggestion();