BUG: "cherry-pick A..B || git reset --hard OTHER"

[Junio C Hamano <gitster@xxxxxxxxx>]

I was burned a few times with this in the past few years, but it did
not irritate me often enough that I didn't write it down.  But I
think this is serious enough that deserves attention from those who
were involved.

A short reproduction recipe, using objects from git.git that are
publicly available and stable, shows how bad it is.

    $ git checkout v2.9.3^0

    $ git cherry-pick 0582a34f52..a94bb68397
    ... see conflict, decide to give up backporting to
    ... such an old fork point.
    ... the git-prompt gives "|CHERRY-PICKING" correctly.

    $ git reset --hard v2.10.2^0
    ... the git-prompt no longer says "|CHERRY-PICKING"

    $ edit && git commit -m "prelim work for backporting"
    [detached HEAD cc5a6a9219] prelim work for backporting

    $ git cherry-pick 0582a34f52..a94bb68397
    error: a cherry-pick or revert is already in progress
    hint: try "git cherry-pick (--continue | --quit | --abort)"
    fatal: cherry-pick failed

    $ git cherry-pick --abort
    ... we come back to v2.9.3^0, losing the new commit!

The above shows two bugs.

 (1) The third invocation of "cherry-pick" with "--abort" to get rid
     of the state from the unfinished cherry-pick we did previously
     is necessary, but the command does not notice that we resetted
     to a new branch AND we even did some other work there.  This
     loses end-user's work.  

     "git cherry-pick --abort" should learn from "git am --abort"
     that has an extra safety to deal with the above workflow.  The
     state from the unfinished "am" is removed, but the head is not
     rewound to avoid losing end-user's work.

     You can try by replacing two instances of

	$ git cherry-pick 0582a34f52..a94bb68397

     with

	$ git format-patch --stdout 0582a34f52..a94bb68397 | git am

     in the above sequence, and conclude with "git am--abort" to see
     how much more pleasant and safe "git am --abort" is.

 (2) The bug in "cherry-pick --abort" is made worse because the
     git-completion script seems to be unaware of $GIT_DIR/sequencer
     and stops saying "|CHERRY-PICKING" after the step to switch to
     a different state with "git reset --hard v2.10.2^0".  If the
     prompt showed it after "git reset", the end user would have
     thought twice before starting the "prelim work".

Thanks.