Fixed issue #1886: Fetch&Rebase should allow automatic branch update if FF

Actually, I am considering to introduce some kind of command script engine into TortoiseGit.
So that we don't write so many post commands here and there.
And perhaps we don't write any post command for some user's specific post command.
But, that is a huge work.

And seems too many lambda function in source code. ha~ ^_^