blame: Notice a wholesale incorporation of an existing file.

The -C option to blame tries to find a section of a preimage
file by running diff against the lines whose origin is still
unknown, and excluding the different parts.  The code however
did not cover the case where the tail part of the section
matched, which we handle for the normal non-move/copy codepath.

This breakage was most visible when preimage file matches in its
entirety and failed to pass blame in such a case.

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