Add support for synchronizing branches

[alexrinass]

As our branch dependency model knows the relationship between all branches and we already provide an update command for a topic branch that synchronizes with its parent, we can take this one step further and provide a sync command that allows users to update all branches once a base branch changes. Keeping branches uptodate as much as possible helps to reduce merge conflicts.

We can detect potential merge conflicts and allow users to only update branches that can be synced without conflicts first.