Teach update-index to read from ls-tree.

Teach update-index to read from ls-tree.

git-update-index --index-info can almost be usable to read from ls-tree
output to update the index (and not the working tree file) to HEAD commit,
but not quite.  It was designed to read from git-apply --index-info
output, and does not want " blob " in ls-tree output.  Accept that as well.

This lets us update "git-checkout <ent> <path>" that used to filter the
extra " blob " string out.  Noted by Luben.

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