diffcore-rename: don't change similarity index based on basename equality

diffcore-rename: don't change similarity index based on basename equality

This implements a suggestion from Johannes.  It uses a separate field in
struct diff_score to keep the result of the file name comparison in the
rename detection logic.  This reverts the value of the similarity index
to be a function of file contents, only, and basename comparison is only
used to decide between files with equal amounts of content changes.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>