feature request: merginal integration

[sfahlberg]

unless I missed some way to switch branches, I think it would be awesome if you could integrate with merginal, or figure out a way improve on what they do

[jreybert]

Branch management in vimagit is in my radar list since the beginning of vimagit. It is quite a a big feature. I'll give a look to merginal, but I had in mind to handle branches internally.

[jreybert]

Duplicated by #141