swarm: calculate linearizedParent while logging

This makes the log sending messages in the order the client will
show it and avoid to calculate linearized parent for each commit.
However, this linearized parent can change over time. In fact, if
a merge happens, the parents of the messages shown can be
modified.

Change-Id: I26372aa52de0e5f89140680a4beaf5fc7ec52163
GitLab: https://git.jami.net/savoirfairelinux/ring-project/-/issues/1282