git add --intent-to-add + git stash "Cannot save the current worktree state"

[Anthony Sottile]

Minimal reproduction

```
git init t
git -C t commit --allow-empty -m 'initial commit'
touch t/a
git -C t add --intent-to-add a
git -C t stash
```

```
+ git init t
Initialized empty Git repository in /private/tmp/t/t/.git/
+ git -C t commit --allow-empty -m 'initial commit'
[master (root-commit) 858132e] initial commit
+ touch t/a
+ git -C t add --intent-to-add a
+ git -C t stash
error: Entry 'a' not uptodate. Cannot merge.
Cannot save the current worktree state
```

Anthony