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

magit-stgit.el

@@ -200,7 +200,10 @@ PROMPT."
         region
         (and use-marks
              (magit-stgit-patches-sorted magit-stgit-marked-patches))
-        (list (or (and use-point (magit-section-when stgit-patch))
+        (list (or (and use-point
+                       (if (fboundp 'magit-section-value-if)
+                           (magit-section-value-if 'stgit-patch)
+                         (magit-section-when stgit-patch)))
                   (and prompt (magit-stgit-read-patch prompt require-match)))))))
 
 ;;; Marking