Document empty commit technique to allow merge commits on already merged branches

[rocketraman]

In some cases, a branch may be merged into an integration branch as part of another topic. Generally, one should explicitly merge in dependent branches first, but in some cases this does not happen.

If so, sometimes it is useful to document the merge of that branch in the first-parent log. However, git will refuse to merge the branch because it will report "Already up to date".

We should document the use of empty commits on the topic branch, that will allow git to merge the topic into an integration branch, despite it already being there via another topic.