Branches 'Whatever' and 'origin/Whatever' have diverged. And branch 'Whatever' may be fast-forwarded.

This error in Atlassian SourceTree seems to happen when one is branching off a branch and the main branch is not yet updated, for example if I were tying to make a Whatever3 branch off of the Whatever2 branch that was splintered from the Whatever branch to begin with and there are checkins yet to pull for Whatever. This is confusing because it wouldn't seem at a glance like Whatever should affect breaking Whatever3 off of Whatever2, but whatever. This may all be because there is no easy way to branch off of Whatever2 without first merging it to Whatever. If you merge to Whatever in the name of this hack and then want to throw away the merge after a commit but not a push, just delete the branch from Branches and then get it again from Remotes. You may need to check the checkbox for Force Delete if SorceTree complains.