graph: make the ahead-behind docs clearer
Explain it in local-upstream branch terms so it's easier to grasp than
with the `one` and `two` naming from the merge-base code.
Rename git_count_ahead_behind -> git_graph_ahead_behind
Moved it into graph.{c,h} which i created for the new "graph"
functions namespace. Also adjusted the function prototype
to use `size_t` and `const git_oid *`.