Hi,

This patch fixes some corner cases of the eshell history command about
reading (-r) and appending (-a).

Recipe:

1. create a sample history file containing some lines, e.g.

    ls

2. emacs -Q --eval "(setq eshell-history-file-name ...)"  -f eshell

3. type following commands:

$ ls
$ history -a
$ cd /tmp
$ history -a

'history -a' doesn't distinguish old history from new history items of
current buffer, and always appends the whole history list whenever it
is called, resulting in a mess of the content of history file:

    ls
    ls
    ls
    history -a
    ls
    ls
    history -a
    cd /tmp/
    history -a

If another eshell buffer or emacs instance is launched before we quit
this eshell buffer, they will read messed history. Thus this patch
changes behavior of 'history -a' from "append current history list to
history file" to "append new history in current buffer to history
file", which is also consistent with bash's 'history -a'.

4. continue to type:

$ (setq eshell-hist-ignoredups t)
$ history -a; history -r
$ history

'history -r', which calls eshell-read-history, doesn't remove
consecutive "ls" duplicates from the history file. Thus this patch
amends eshell-read-history to make it respect the
eshell-hist-ignoredups option (both t and 'erase).

Since 'history -r' replaces current history list, which is actually
equivalent to bash's 'history -c; history -r', I have updated the help
text. Maybe it should be split into two commands, i.e. adding 'history
-c'?


Best,

--
Liu Hui