appview: revert mergeCheck logic

Signed-off-by: oppiliappan <[e-mail]>

appview/pulls/pulls.go

···
 	patch := pull.LatestPatch()
 	if pull.IsStacked() {
 		// combine patches of substack
-		subStack := stack.StrictlyBelow(pull)
+		subStack := stack.Below(pull)
 		// collect the portion of the stack that is mergeable
 		mergeable := subStack.Mergeable()
 		// combine each patch