merge-recursive: make sure when we say we abort that we actually abort

merge-recursive: make sure when we say we abort that we actually abort

In commit 65170c07d4 ("merge-recursive: avoid incorporating uncommitted
changes in a merge", 2017-12-21), it was noted that there was a special
case when merge-recursive didn't rely on unpack_trees() to enforce the
index == HEAD requirement, and thus that it needed to do that enforcement
itself.  Unfortunately, it returned the wrong exit status, signalling that
the merge completed but had conflicts, rather than that it was aborted.
Fix the return code, and while we're at it, change the error message to
match what unpack_trees() would have printed.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>