Eshell output filter

[Benjamín Buccianti]

Hello there!

I've encountered a problem with eshell and apk (package manager of Alpine 
Linux).

When running an update, I see some chars that I think are used to delete the 
line (behaviour that you can see on any terminal emulator).

For example:

```
~ $ doas apk -U upgrade
fetch http://dl-cdn.alpinelinux.org/alpine/edge/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/community/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/testing/x86_64/APKINDEX.tar.gz
%1b7  0%                                             %1b8OK: 3694 MiB in 819 
packages
```

The expected outcome it's to see only this:

```
~ $ doas apk -U upgrade
fetch http://dl-cdn.alpinelinux.org/alpine/edge/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/community/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/testing/x86_64/APKINDEX.tar.gz
OK: 3694 MiB in 819 packages
```

I think we don't see the 0% because it occurs really fast. But in some cases, 
you can see the progress of the bar changing from 0% to 100%. I think ^[8 it's 
used to erase the line because in other cases you can see output like this:

```
~ $ doas apk add emacs
(1/2) Installing emacs-nox (27.1-r1)
%1b7  0%                    %1b8%1b7 97% 
########################################### %1b8%1b7 98% 
########################################### %1b8%1b7 99% 
########################################### %1b8(2/2) Installing emacs (27.1-r1)
%1b7 99% ########################################### %1b8%1b7100% 
############################################%1b8Executing 
busybox-1.32.0-r3.trigger
Executing gtk-update-icon-cache-2.24.32-r2.trigger
OK: 3802 MiB in 821 packages
```
(output reduced for the sake of understanding, but you can see the whole 
sequence)

So, I figured out that eshell uses a eshell-handle-control-codes to detect 
control codes.

This is a proposed alternative but I'm not really that well versed on emacs and 
on terminal control codes so, I post here with the intention of see what you 
guys think and with the desire of improve with your help.

diff --git a/lisp/eshell/esh-mode.el b/lisp/eshell/esh-mode.el
index db5fddb2aa..ed5c864eda 100644
--- a/lisp/eshell/esh-mode.el
+++ b/lisp/eshell/esh-mode.el
@@ -999,6 +999,14 @@ eshell-handle-control-codes
           (beep))
          ((eq char ?\C-h)
           (delete-region (1- (point)) (1+ (point))))
+         ((and (eq char ?\)
+               (memq (char-after (1+ (point)))
+                    '(?\8)))
+         (let ((end (1+ (point))))
+           (beginning-of-line)
+           (delete-region (point) end)
+           (delete-char 1))
+         (forward-char))
          (t
           (forward-char)))))))
 

Thanks!