Please add a warning when attempting to merge the base of a stack

[huntertdiamond]

I had a four stacked PR.

I pressed the merge button on the base of the stack, which only merged the base PR, while I expected it to merge all PRs in the stack.

Please add something a bit more obvious than the small number inside the merge button, to indicate that pressing merge will not squash the entire stack and merge.

Aside from that, everything has been great!

thanks