When Bazaar tries to push one branch onto another, it requires that the destination branch must be ready to receive the source branch. If this isn’t the case, then we say that the branches have diverged. Branches are considered diverged if the destination branch’s most recent commit is one that has not been merged (directly or indirectly) by the source branch. To recover from diverged branches, one must merge the missing revisions into the source branch.
This situation commonly arises when using a centralized workflow with local commits. If someone else has committed new work to the mainline since your last pull and you have local commits that have not yet been pushed to the mainline, then your local branch and the mainline have diverged.
Discovering What Has Diverged
The bzr missing command is used to find out what revisions are in another branch that are not present in the current branch, and vice-versa. It shows a summary of which extra revisions exist in each branch. If you want to see the precise effects of those revisions, you can use bzr diff --old=other_branch to show the differences between other_branch and your current branch.
The solution is to merge the revisions from the mainline into your local branch. To do so, use bzr merge to get the new revisions from the mainline. This merge may result in conflicts if the other developer’s changes overlap with your changes. These conflicts should be resolved before continuing. After any conflicts have been resolved, or even if there were no conflicts, Bazaar requires that you explicitly commit these new revisions to your local branch. This requirement gives you an opportunity to test the resulting working tree for correctness, since the merged revisions could have made arbitrary changes. After testing, you should commit the merge using bzr commit. This clears up the diverged branches situation. Your local branch can now be pushed to the mainline.