checkout: do not make a temporary copy of symlink target.

checkout: do not make a temporary copy of symlink target.

If the index records an insanely long symbolic link, copying
into the temporary would overflow the buffer (noticed by Mark
Wooding).

Because read_sha1_file() terminates the returned buffer with NUL
since late May 2005, there is no reason to copy it anymore.

Signed-off-by: Junio C Hamano <junkio@cox.net>