comments: don't leave stray "Add Another Comment" buttons after cancelling adding comments on a new line

2e72d2d16a0f introduced an invisible "submitting" div above comment forms. That
div was included when counting the number of comments on a line, and it thus
broke the logic for removing the last DOM elements after cancelling comments on
a new line.

To fix that, exclude the submitting div in the selector used for counting
comments.