unpack-trees: preserve the index file version of original
unpack-trees: preserve the index file version of original

Otherwise "git checkout $other_branch" (or even "git checkout HEAD")
would end up writing the index out in the default format.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git