rebase -i: Retain user-edited commit messages after squash/fixup conflicts

rebase -i: Retain user-edited commit messages after squash/fixup conflicts

When a squash/fixup fails due to a conflict, the user is required to
edit the commit message.  Previously, if further squash/fixup commands
followed the conflicting squash/fixup, this user-edited message was
discarded and a new automatically-generated commit message was
suggested.

Change the handling of conflicts within squash/fixup command series:
Whenever the user is required to intervene, consider the resulting
commit to be a new basis for the following squash/fixups and use its
commit message in later suggested combined commit messages.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>