bug#55838: 29.0.50; Eshell string-split subscript indexing splits too much

[Jim Porter <jporterbugs@gmail.com>]

 From "emacs -Q -f eshell":

   M-: (setq foo "a\nb:c")

   ~ $ echo $foo
   a
   b:c
   ~ $ echo $foo[: 0]
   ("a" "b")

The first command is normal, and just shows that Eshell outputs the 
string with no manipulation. In the second command, we split the string 
on ":" and get the 0th element. However, that gets split *again* (on 
newlines) and returns a list.

I think this is overly aggressive. It's due to `eshell-apply-indices' 
calling `eshell-convert' on the split element(s) of the string. However, 
`eshell-convert' is primarily designed to turn output from external 
command line programs into a Lispy form (so it splits by line to make a 
list, among other things). This would normally happen when doing 
something like this:

   ~ $ echo ${cat some-file.txt}
   ("line 1" "line 2" ...)

In the original case above, I think the split-subscript operator [: 0] 
should only be doing the one thing the user requested: split on ":" and 
get the 0th element.

Patch forthcoming momentarily. Just getting a bug number.