replace gitcmd with go-git

[lukas016]

Hi,

could be possible replace https://github.com/wadackel/go-gitcmd with https://github.com/go-git/go-git?

Main reason is dependency to git command. It looks as external dependency which can be problematic in future because gitcmd wasn't updated 5 years and git is still develop.

Using golang implementation of git can improve re-usability of app between multiple different system.