stash pop results in conflict after stash --keep-index with the same file in index and working directory

["Jakub Dąbek" <jakub.dabek@gmail.com>]

I was trying to write a pre-commit hook for testing using stash to
keep irrelevant changes, but pop couldn't apply changes automatically
when the same file with different changes was in index and working
directory at the same time.

Reproduction:

  mkdir git-stash-test && cd git-stash-test
  git init
  echo foo > foo.txt
  git add . && git commit -m 'init'
  echo foo2 > foo.txt
  git add foo.txt
  echo foo3 > foo.txt
  git status # (*)
  git stash push --keep-index
  git status
  # do testing
  git stash pop

Expected result: same state as before push (*); actual result:

  Auto-merging foo.txt
  CONFLICT (content): Merge conflict in foo.txt
  The stash entry is kept in case you need it again.

--index option doesn't help when used with pop.

My assumption is that pop should be the reverse of push when nothing
has changed in between, but I don't know if this is a bug or
unexpected behaviour. (git version 2.26.2)