bug#10835: 24.0.93; bidi-paragraph-direction slows down Shell mode

[Chong Yidong <cyd@gnu.org>]

See thread discussion here:

http://lists.gnu.org/archive/html/emacs-devel/2012-02/msg00517.html

To recap:

1. emacs -Q
2. M-x shell RET
3. ls /usr/share/doc    (Prints about 2000 lines on my computer)
4. Repeat step 6-7 times (e.g. M-p RET)

The insertion of the shell output is significantly slower than in Emacs
23.  If you set `bidi-paragraph-direction' to `left-to-right', the
slowdown mostly disappears.

If you scroll the shell buffer, it is also more sluggish when
`bidi-paragraph-direction' is nil.

Another observation: if you save the *shell* buffer to a file and visit
the file in another Emacs instance, there is no sluggishness if the file
is opened in (say) text mode.  Maybe something in Shell mode is
interacting badly with the bidi code, but I don't know what.



In GNU Emacs 24.0.93.7 (x86_64-unknown-linux-gnu, GTK+ Version 3.2.0)
 of 2012-02-15 on furball
Windowing system distributor `The X.Org Foundation', version 11.0.11004000
Configured using:
 `configure '--with-x-toolkit=gtk3' 'CFLAGS=-g''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: en_US.UTF-8
  value of $LC_CTYPE: en_US.UTF-8
  value of $LC_MESSAGES: en_US.UTF-8
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Emacs-Lisp