Stash apply requires clean index

[wmertens]

Reproduction steps

try to apply a stash when index is dirty. This is specifically prevented by

libgit2/src/stash.c

Line 900 in 66137ff

if ((error = ensure_clean_index(repo, repo_index)) < 0)

which was added in #3259

However, git allows this:

3066 $ git init
Initialized empty Git repository in /home/wmertens/testgit/.git/
3067 $ touch bar
3071 $ git commit  -a -m 'hi'
[master (root-commit) b37d552] hi
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 bar
3082 $ echo hi  > foo
3084 $ git add foo
3088 $ git stash
Saved working directory and index state WIP on master: b37d552 hi
3089 $ echo boom  > foo
3091 $ git add foo
3093 $ git stash apply
CONFLICT (add/add): Merge conflict in foo
Auto-merging foo
3094 $ git status
On branch master
Unmerged paths:
  (use "git restore --staged <file>..." to unstage)
  (use "git add <file>..." to mark resolution)
        both added:      foo
3095 $ cat foo
<<<<<<< Updated upstream
boom
=======
hi
>>>>>>> Stashed changes

IMHO libgit should behave like git, and the clean check should not be done.