bug#40407: [PATCH] slow ENCODE_FILE and DECODE_FILE

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Sat, 4 Apr 2020 00:32:21 +0200
> Cc: 40407@debbugs.gnu.org
> 
> Since string mutation is so rare, I doubt it has caused any real trouble. Now, do we fix it by inverting the sense of the argument, or by renaming it to COPY? I'm fairly neutral, but there are arguments in either way, both in terms of performance and correctness. And what about internal calls to code_convert_string?
> 
> There are 193 calls to {encode, decode}-coding-string in the Emacs tree, and only 14 of them pass a non-nil value to NOCOPY. I'd be inclined to keep the semantics but rename the argument to COPY, on the grounds that no-copy is a better default; then change those 14 calls to pass nil instead, since that obviously was the intent.

After looking at this for some time, I think the problem is rarely if
ever seen.  The only function which has the NOCOPY sense inverted is
code_convert_string, and it only does that when the CODING_SYSTEM
argument is nil, which should almost never happen.  So I think it's OK
to change code_convert_string on master to use NOCOPY in its correct
sense.