Make HEAD reference a branch by drawing an arrow

[agutenkunst]

It would be useful for "HEAD" to draw a arrow to the branch it is referencing.
This would make

a bit more clearer.

Maybe related to #1

[oliviercailloux]

Relatedly, note that after git branch newbranch, the HEAD box is in between the two branches, but after git checkout, the HEAD box is back to the bottom of the stack. So the HEAD relative position in the stack of pointers does not seem to indicate anything relevant. If this is intended, then it would be better to make HEAD always at the bottom.