Add ability to bulk rebase

[Mingun]

When working on some project, I prefer to keep all my branches based on the latest master. On some projects I also have dozen of branches. Each time when master is updated I have to manually switch branch and rebase it. It would be nice if I can do a bulk rebase over specified commit.

The implementation might look as:

• In the context menu of the commit select option Rebase branches onto this...
• The window with the list of branches is appeared where you can check which branches you want to rebase. By default it should remember the previous choice and include new branches to the list
• Ok button will start rebase of all branches

In most cases my branches rebased without conflicts, so this feature will greatly improve my experience. If conflict appeared, it should be solved in the usual way, and then bulk rebase should continue. Idealy, if conflict would not stop rebasing of other branches in parallel, but I think, git wouldn't allow that.

The similar feature I requested in TortoiseGit.