Setting bidi-paragraph-direction in comint modes

[Chong Yidong]

Any objections to setting bidi-paragraph-direction to `left-to-right' in
Comint and derived modes?  The bidi code appears to lead to a
performance regression.  You can observe this with

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

As the buffer gets to about 8000 lines, redisplay slows to a crawl.
Setting bidi-paragraph-direction to `left-to-right' fixes this (mostly;
it's still a touch slower than in Emacs 23).  AFAICT, bidi paragraphs
don't make sense in Comint-like modes anyway.