Rebase current branch onto another branch

[tomerstav]

Added a new command 'Rebase Branch Onto...' in the git command options that lets you select a branch to rebase your current branch onto. Behind the scenes git checkouts the selected branch and runs git rebase [previous branch].

This PR fixes #108725

[joaomoreno]

Although this looks polished, it's doing the wrong thing. It checks out the target branch and rebases that to the current branch. It should instead just do git rebase TARGET_BRANCH.

[tomerstav]

@joaomoreno Looks good now? Any other changes you would like?

[joaomoreno]

Yes, we probably should remove all onto references from the command names, etc, so not to confuse users who know what --onto does.

Also, when providing a choice picker to the user, we should automatically highlight the branch's upstream, if defined.

[joaomoreno]

Thanks!