was rebased since last fetched, the rebase uses that information
to avoid rebasing non-local changes.
+
-When preserve, also rebase the current branch on top of the upstream
-branch, but pass `--preserve-merges` along to `git rebase` so that
-locally created merge commits will not be flattened.
+When set to preserve, rebase with the `--preserve-merges` option passed
+to `git rebase` so that locally created merge commits will not be flattened.
+
When false, merge the current branch into the upstream branch.
+