http://marklodato.github.io/visual-git-guide/index-zh-cn.html#merge
http://marklodato.github.io/visual-git-guide/index-zh-cn.html#rebase
http://marklodato.github.io/visual-git-guide/index-zh-cn.html#detached
http://en.wikipedia.org/wiki/Three-way_merge
http://www.kernel.org/pub/software/scm/git/docs/git-rebase.html#_interactive_mode