show newer commits if on detached head

Right now the log starts from HEAD - no matter if that's a tip of a branch or not.

For #81 we need to do the revwalk from every tip and come up with a smart data structure that allows us to draw the correct graph without storing the whole graph in memory.

Before tackling #81 we can at least [determine](https://libgit2.org/libgit2/#HEAD/group/graph/git_graph_descendant_of) if we are on a detached head and then iterate the branches whether one is a descendent of our current HEAD and then start from the branch tip and visualize where HEAD is in the log.