commit: allow --amend to reuse message from another commit

commit: allow --amend to reuse message from another commit

After tentatively applying a patch from a contributor, you can get a
replacement patch with corrected code and unusable commit log message.
In such a case, this sequence ought to give you an editor based on the
message in the earlier commit, to let you describe an incremental
improvement:

    git reset --hard HEAD^ ;# discard the earlier one
    git am <corrected-patch
    git commit --amend -c HEAD@{1}

Unfortunately, --amend insisted reusing the message from the commit
being amended, ignoring the -c option.  This corrects it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>