diff --cached: do not borrow from a work tree when a path is marked as assume-unchanged

diff --cached: do not borrow from a work tree when a path is marked as assume-unchanged

When the index says that the file in the work tree that corresponds to the
blob object that is used for comparison is known to be unchanged, "diff"
reads from the file and applies convert_to_git(), instead of inflating the
object, to feed the internal diff engine with, because an earlier
benchnark found that it tends to be faster to use this optimization.

However, the index can lie when the path is marked as assume-unchanged.
Disable the optimization for such paths.

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