Document Git 2.18 feature `--rebase-merges`

[rocketraman]

Git 2.18 has an option for interactive rebase called --rebase-merges, which will eventually replace the clunky --preserve-merges. This capability can be used to rebase a topic with merges interactively, and can therefore be very useful for topic-based development with gitworkflow.

See https://git-scm.com/docs/git-rebase#_rebasing_merges

Add information to the docs about how this option can be used e.g.:

• Redoing merges of other topics (for example, if that other topic was itself rebased)
• Splitting topic branches up into separate topics
• Adding additional merge commits to the topic without rebuilding it with reset, merge, and cherry-pick