fix: Avoid displaying spurious conflicts #371

could replace the `iter::Once` in the return type signature, but I think
it's actually nice, since it signifies that in that case, we return the
`MergedTree:Conflict`, _once_

all we needed was to have `{first,second}_rev` inside the closure

doesn't compile, since the types of the last branch is technically
different from the rest

Fixes #356.
Not including the original case as integration test case because I
didn't manage to minimize it sufficiently. I think the unit test is
enough.