document and fix findincoming
- add documentation about what the function does, notably
the fact that it updates 'base'
- transform the workflow to a more simple 'if elif elif else'
- do not call remote.branches if not necessary
- some nodes where missing in 'base' (from what I understand,
if the root of a branch is missing but one parent is present,
the parent should be in 'base')
- add a testcase for an incorrect outgoing that is fixed by
this cset
- add a testcase for an empty group bug, it needs fixing