-# Check if we are already based on $onto, but this should be
-# done only when upstream and onto are the same.
-mb=$(git-merge-base "$onto" "$branch")
-if test "$upstream" = "$onto" && test "$mb" = "$onto"
+# Check if we are already based on $onto with linear history,
+# but this should be done only when upstream and onto are the same.
+mb=$(git merge-base "$onto" "$branch")
+if test "$upstream" = "$onto" && test "$mb" = "$onto" &&
+ # linear history?
+ ! git rev-list --parents "$onto".."$branch" | grep " .* " > /dev/null