comint-insert-previous-argument counts args from start or from end

comint-insert-previous-argument counts args from start or from end

This function is invoked in shell-mode by the user, and is meant to
emulate what M-. does in zsh and bash: it inserts an argument from a
previous command.  Without a prefix argument, it inserts the last arg
from the previous command; with an argument INDEX, it inserts the
INDEX-th argument.  bash counts from the start, while zsh counts from
the end.  This patch adds a variable
`comint-insert-previous-argument-from-end' that emulates the zsh
behavior if non-nil.

* lisp/comint.el (comint-arguments): can take in negative arguments to count
  from the end, same as indexing in python.
  (comint-insert-previous-argument): if
  comint-insert-previous-argument-from-end is non-nil, INDEX counts
  arguments from the end; if nil, from the beginning
  (Bug#25271)
* etc/NEWS: Document this.