Reword git-am 3-way fallback failure message.

Reword git-am 3-way fallback failure message.

When the blobs recorded on the index lines in the patch as pre-image
blobs are not found in the repository, "git-am" punted saying
that the index line does not record anything useful.  This was not
clear enough -- the index line does have something useful but the
problem was that it was not useful in _that_ repository.

Reword the message as Francis Moreau suggests.

Signed-off-by: Junio C Hamano <junkio@cox.net>