t7001: fix "git mv" test

t7001: fix "git mv" test

The test assumed that we can keep the cached stat information fresh across
rename(2); many filesystems however update st_ctime (and POSIX allows them
to do so), and that assumption does not hold.

We can explicitly refresh the index for the purpose of these tests.  The
only thing we are interested in is the staged contents and the mode bits
are preserved across "git mv".

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