Bug: git-stash fails for new file not staged for commit

[Claas Augner <claas.augner@lsp.net>]

 > What did you do before the bug happened?

I tried to stash a new file that I added with `git add --intent-to-add`:

```
$ touch foo
$ git add --intent-to-add foo
$ git stash
```


 > What did you expect to happen?

The file should have been stashed.


 > What happened instead?

The stash failed with the following error message:

```
error: Entry 'foo' not uptodate. Cannot merge.
Cannot save the current worktree state
```


 > What's different between what you expected and what actually happened?

Instead of stashing the file, the stash fails with an error message.


 > Anything else you want to add:

Background: https://github.com/okonet/lint-staged/issues/990


[System Info]
git version:
git version 2.32.0
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 5.4.72-microsoft-standard-WSL2 #1 SMP Wed Oct 28 23:40:43 
UTC 2020 x86_64
compiler info: gnuc: 9.3
libc info: glibc: 2.31
$SHELL (typically, interactive shell): /usr/bin/zsh


[Enabled Hooks]
applypatch-msg
pre-applypatch
post-applypatch
pre-commit
prepare-commit-msg
commit-msg
post-commit
pre-rebase
post-checkout
post-merge
pre-push
pre-receive
update
post-receive
post-update
push-to-checkout
pre-auto-gc
post-rewrite
sendemail-validate

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