[patch v2] switching and creating branch, merging and rebasing with Emacs

I've a new version, in the form of a patch series that follow: first I add some function for asking user commitish then some code for checkouting commitish and branching, mostly following the idea of Sergei, then some code for merging.

I've look to add rebase, but when thinking about how to do about conflict and continued rebasing, I feel lazy, and didn't try. As someone a good idea for the interface?