fix: support magit 2.90

The magit-section-when function has been deprecated in magit 2.90 [1]
and was removed in March.

[1]: https://github.com/magit/magit/blob/c7ab6931c61e63db02edd2f9ad3688312936cf0b/docs/RelNotes/2.90.0.org#breaking-changes

Fixes: #16
Suggested-by: cpg314

Author: jpgrayson