rebase: help users when dying with `preserve-merges`

rebase: help users when dying with `preserve-merges`

Git would die if a "rebase --preserve-merges" was in progress.
Users could neither --quit, --abort, nor --continue the rebase.

Make the `rebase --abort` option available to allow users to remove
traces of any preserve-merges rebase, even if they had upgraded
during a rebase.

One trigger case was an unexpectedly difficult to resolve conflict, as
reported on the `git-users` group.
(https://groups.google.com/g/git-for-windows/c/3jMWbBlXXHM)

Other potential use-cases include git-experts using the portable
'Git on a stick' to help users with an older git version.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Junio C Hamano <gitster@pobox.com>