When creating a Pull request (with Merge to close the source branch ) into a branch that was already merged or there is an existing PR request for it, we should show a UI message indicating that the target branch is either merged, set up to be merged by another PR or the branch was deleted. We should Disable the Merge button on that PR.

Example:
have
master branch
branch 1
branch 2
create a PR 1: branch 2 -> Master
create a PR 2: branch 1 -> branch 2
this should be giving a warning that branch 2 is either merged already, is scheduled to be merged or was deleted