Last flow detection not working properly when last backflow is from a different branch than the one we're forward flowing to

[dkurepa]

In dotnet/dotnet#7141, we weren't using the correct direction algorithm because we re not detecting that the last backflow was coming from a different branch than the one we were forward flowing to.
This detection happens in

arcade-services/src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrCodeflower.cs

Line 450 in db1476c

if (!headBranchExisted && !await vmr.IsAncestorCommit(lastBackflow.VmrSha, currentVmrSha))

but it failed because in a local flow, the head branch always exists.

I can't remember why we added it at the time of writing this, but I know there was a bug where an existing FF PR couldn't get an opposite direction flow if the PR already existed.

Apparently that caused this bug now