diffcore-rename: fall back to -C when -C -C busts the rename limit

diffcore-rename: fall back to -C when -C -C busts the rename limit

When there are too many paths in the project, the number of rename source
candidates "git diff -C -C" finds will exceed the rename detection limit,
and no inexact rename detection is performed.  We however could fall back
to "git diff -C" if the number of modified paths is sufficiently small.

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