Staging from hunk preview can stage change into wrong file

[airblade]

1. Open two different files.
2. Change the first file and preview the change.
3. Go to second window and then to hunk preview.
4. Stage the hunk.
5. Expected: change is staged into first file. Actual: change is staged into second file.

This is because the code uses wincmd p to return to the parent window when staging from a preview.

Solution: store the parent window in a buffer variable in the hunk preview and use that when returning to the original window.