conversationrepository: avoid recursive merge

There is no need as we always merge two branches so we can avoid
a lot of recursive operations. Moreover, this seems to cause some
weird locks

To be honest I'm not sure about the usage of this argument, doc is
still a bit unclear:

    Maximum number of times to merge common ancestors to build a
    virtual merge base when faced with criss-cross merges.
    When this limit is reached, the next ancestor will simply be used
    instead of attempting to merge it. The default is unlimited.

GitLab: #871
Change-Id: I9c22208626e7f6ff489e91b72a580238a6137499