tests: add an explicit destination in some rebase tests As we will make 'rebase' behave more like 'merge', it will no longer pick 'max(branch(.))' as the default destination. We have to hard code the expected destination is multiple tests where it matters. After a careful inspection none of theses tests really cares about the default destination behavior and just omitted one out of laziness.

tests: remove third head in some of the 'rebase-parameters' tests These tests do not care about that extra branch at all. In future changeset we will make rebase behave like merge and abort in case of an ambiguous destination (eg: multiple other heads) and that extra branch will make the command invocation breaks. We preventively remove this extra branch from the relevant tests in an independant changeset to reduce noise and increase confidence in the final change.