diff Porcelain: do not disable auto index refreshing on -C -C

diff Porcelain: do not disable auto index refreshing on -C -C

When we enabled the automatic refreshing of the index to "diff" Porcelain,
we disabled it when --find-copies-harder was asked, but there is no good
reason to do so.  In the following command sequence, the first "diff"
shows an "empty" diff exposing stat dirtyness, while the second one does
not.

    $ >foo
    $ git add foo
    $ touch foo
    $ git diff -C -C
    $ git diff -C

This fixes the inconsistency.

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