file loss by git stash push --staged --keep-index

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Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)
Running the following script:

#!/bin/bash
git init workDir
cd workDir
git commit -m first --allow-empty
echo b > b
git add b
GIT_TRACE=TRUE git stash push --staged --keep-index
git stash pop
cat b

What did you expect to happen? (Expected behavior)
I expect file `b` to be present after running the script.
If I run the script with only `git stash push --staged` or 
`git stash push --keep-index`, the file is present afterward.

What happened instead? (Actual behavior)
The file is not present after running the script.
`cat b` will report that the file does not exist.