Add --allow-empty argument for 'stg commit'

The --allow-empty argument was missing as an option from the 'stg
commit' popup. Add it as '-e'.

Signed-off-by: Toke Høiland-Jørgensen <[email protected]>

Author: tohojo

magit-stgit.el

@@ -391,6 +391,7 @@ minibuffer as well."
   :man-page "stg-commit"
   ["Arguments"
    ("-a" "Commit all applied patches" "--all")
+   ("-e" "Allow empty patches to be committed" "--allow-empty")
    ("-n" "Commit the first N patches from the bottom up" "--number="
     :reader (lambda (prompt _initial-input history)
               (number-to-string (read-number prompt nil history))))]