apply: do not read from the filesystem under --index

apply: do not read from the filesystem under --index

We currently read the preimage to apply a patch from the index only
when the --cached option is given.  Do so also when the command is
running under the --index option.  With --index, the index entry and
the working tree file for a path that is involved in a patch must be
identical, so this should not affect the result, but by reading from
the index, we will get the protection to avoid reading an unintended
path beyond a symbolic link automatically.

Signed-off-by: Junio C Hamano <redacted>