bug#7485: 23.2; Fix removing unrecognized ANSI sequences in ansi-color-apply

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Leo <sdl.web@gmail.com> writes:

> Unrecognized control sequences also begin with char \033. The FRAGMENT
> part of ansi-color-context wasn't supposed to contain actual text,
> otherwise they might get lost. It should always be a fragment of a whole
> ansi control sequence if non-nil.
>
> Let me show how this stops eshell prompt from showing.
>
> Assume ansi-color-apply is in eshell-preoutput-filter-functions. Suppose
> the output of shell command 'ack linux' ends with an unrecognized
> control sequence ^[[K. The whole output string that goes through
> ansi-color-apply is
>
>    output of 'ack linux' + 'eshell prompt'.
>
> After 'ack linux' finishes, the FRAGMENT of ansi-color-context becomes:
>
> #("^[[K\n~/.emacs.d $ " 4 17
>    (read-only t face eshell-prompt rear-nonsticky
>               (face read-only)))
>
> The string after being processed by ansi-color-apply is thus without an
> eshell prompt.

I think this patch is no longer applicable now that I've pushed
[1:35ed01dfb3], ansi-color-apply now skips over every escape sequence,
not just color escape sequences.  Although I'm unable to produce the
problem of missing prompts even before my patch (e.g., in 25.2):

    ~/src $ printf 'ack linux\e[K\n'
    ack linux
    ~/src $ 

[1: 35ed01dfb3]: 2017-07-03 10:09:40 -0400
  Fix and simplify ansi escape detection (Bug#21381)
  
http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=35ed01dfb3f811a997e26d843e9971eb6b81b125