[Bug?] "am --abort" broken?

[Junio C Hamano <gitster@pobox.com>]

This is somewhat minor but will confuse people who do not know how
to manually recover from.  Start "am" without giving an input file
by mistake:

    $ git am ;# no input file
    ^C

At this point, you are still in "am" session; completion will tell
you that you are in a middle of "am/rebase".  A more recent way to
get out of this is to use "am --abort", but you get this:

    $ git am --abort
    Resolve operation not in progress, we are not resuming.

"git am" from 'maint' will help you get out of it, or you can do it
the old fashioned way:

    $ rm -fr .git/rebase-apply

I think this is a recent breakage after merging the topic to add
"rebase --autostash", but I haven't looked into it yet.