git-rebase-interactive: do not squash commits on abort

git-rebase-interactive: do not squash commits on abort

If git rebase interactive is stopped by "edit" command and then the user
said "git rebase --continue" while having some stage changes, git rebase
interactive is trying to amend the last commit by doing:
  git --soft reset && git commit

However, the user can abort commit for some reason by providing an empty
log message, and that would leave the last commit undone, while the user
being completely unaware about what happened. Now if the user tries to
continue, by issuing "git rebase --continue" that squashes two previous
commits.

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>