Improve read/append behavior of eshell history command

Improve read/append behavior of eshell history command

* lisp/eshell/em-hist.el (eshell-hist--new-items): New variable.
(eshell-hist-initialize): Initialize 'eshell-hist--new-items' to 0.
(eshell/history): Change the behavior of 'history -a' to "append new
history in current buffer to history file".  Clarify the help text of
'history -r'.
(eshell-add-input-to-history): Increase counter of new history items.
(eshell-read-history): Respect 'eshell-hist-ignoredups' option.
(eshell-write-history): If the optional argument APPEND is non-nil,
appending new history items rather than the whole history.

* test/lisp/eshell/em-hist-tests.el (em-hist-test/history-append)
(em-hist-test/history-read): New tests (bug#66768).