Stash apply requires clean index

Reproduction steps

try to apply a stash when index is dirty. This is specifically prevented by https://github.com/libgit2/libgit2/blob/66137ff6ea9e516e0fa840134393d5a81d5b86e9/src/stash.c#L900 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.

Start in src/stash.c around the clean-index check at the cited line, then compare the behavior with the provided Git reproduction. Remove the restriction only if the existing stash-apply flow can handle the dirty index; done means applying the stash permits the conflict behavior shown by Git.