git:perm-stash: Use stash's ancestor as commit_ref

The stash does not necessarily apply to HEAD.

.config/zsh/autoload/git/git-perm-stash

@@ -11,7 +11,7 @@ git rev-parse
 
 idx="${1:-0}"
 stash_ref="$(git rev-parse stash@{$idx} | cut -c-$REF_LEN)"
-commit_ref="$(git rev-parse HEAD | cut -c-$REF_LEN)"
+commit_ref="$(git rev-parse stash@{$idx}~ | cut -c-$REF_LEN)"
 
 git stash show -up "$idx" > \
 	"${2:-$DIR}/${PWD//\//__}__${commit_ref}__$stash_ref.diff"