git read-tree doesn't accept tree-ish

[Nikita Bobko <nikitabobko@xxxxxxxxx>]

```
git init
echo foo > foo
git add --all
git commit -m 'foo'

echo bar > foo
git add --all
git commit -m 'bar'

git read-tree HEAD~:foo
```
EXP: Read information about file `foo` into index
ACT: `fatal: failed to unpack tree object HEAD~:foo`

I expect it to work because manpage says that `git-read-tree` accepts
`<tree-ish>`