bug#56682: Fix the long lines font locking related slowdowns

[Gregory Heytings]

> > So the only remaining question is whether it is necessary to recompute 
> > narrowed_begv and narrowed_zv in init_iterator:
>
> I tend to think we should, but let me think about this some more.

Okay, I'll wait for your feedback.

Wouldn't it make sense to also limit the portion of the buffer to which 
pre-/post-command-hook have access (see below)?

With that patch, I was able to open and edit a file with a single 50 GB 
(!) line, in js-mode.  Does that still not qualify as "arbitrarily large"?

I compared that with a 50 GB JSON file with 80 character wide lines. 
With that file it was necessary to disable font-lock-mode, which took too 
much time.  Apart from that, I did not see any significant performance 
differences while editing the file, compared to the single line one.

diff --git a/src/keyboard.c b/src/keyboard.c
index 2863058d63..ce529222a3 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1461,7 +1461,22 @@ command_loop_1 (void)
       }
       Vthis_command = cmd;
       Vreal_this_command = cmd;
-      safe_run_hooks (Qpre_command_hook);
+
+      if (current_buffer->long_line_optimizations_p)
+       {
+         specpdl_ref count = SPECPDL_INDEX ();
+         struct window *w = XWINDOW (selected_window);
+         ptrdiff_t begv = get_narrowed_begv (w, PT);
+         ptrdiff_t zv = get_narrowed_zv (w, PT);
+         if (!begv) begv = BEGV;
+         Fnarrow_to_region (make_fixnum (begv), make_fixnum (zv), Qt);
+         safe_run_hooks (Qpre_command_hook);
+         unbind_to (count, Qnil);
+       }
+      else
+       {
+         safe_run_hooks (Qpre_command_hook);
+       }

       already_adjusted = 0;

@@ -1513,7 +1528,21 @@ command_loop_1 (void)
           }
       kset_last_prefix_arg (current_kboard, Vcurrent_prefix_arg);

-      safe_run_hooks (Qpost_command_hook);
+      if (current_buffer->long_line_optimizations_p)
+       {
+         specpdl_ref count = SPECPDL_INDEX ();
+         struct window *w = XWINDOW (selected_window);
+         ptrdiff_t begv = get_narrowed_begv (w, PT);
+         ptrdiff_t zv = get_narrowed_zv (w, PT);
+         if (!begv) begv = BEGV;
+         Fnarrow_to_region (make_fixnum (begv), make_fixnum (zv), Qt);
+         safe_run_hooks (Qpost_command_hook);
+         unbind_to (count, Qnil);
+       }
+      else
+       {
+         safe_run_hooks (Qpost_command_hook);
+       }

       /* If displaying a message, resize the echo area window to fit
         that message's size exactly.  Do this only if the echo area