GitGraph: support a layout more « ref-oriented »

[gbonnefille]

Currently, GitGraph is just wonderful to explain a Git workflow.

But, when studying Git, the representation of the relation between commit and branches can be misleading. Having branches displayed in background as railway, one can imagine that commits belong to branches. While it is wrong technically. In Git, the whole history is supported by commits their selves and branches, like tags, are only a labelled way to access this history.

It would be nice to be able to configure the layout of the gitGraph in order to have something similar to https://www.nicoespeon.com/gitgraph.js/#6 where branches are also displayed as label, like tags.