API proposal to retrieve both the ahead/behind count, and the actual commits. `git_commit_list` would be an opaque type containing a `git_vector`

gistfile1.c

typedef struct git_commit_list git_commit_list;

/**
 * Count the number of unique commits between two commit objects
 *
 * There is no need for branches containing the commits to have any
 * upstream relationship, but it helps to think of one as a branch and
 * the other as its upstream, the `ahead` and `behind` values will be
 * what git would report for the branches.
 *
 * @param ahead number of unique from commits in `upstream`
 * @param behind number of unique from commits in `local`
 * @param repo the repository where the commits exist
 * @param local the commit for local
 * @param upstream the commit for upstream
 */
GIT_EXTERN(int) git_graph_ahead_behind(
  git_commit_list **ahead,
	git_commit_list **behind,
	git_repository *repo,
	const git_oid *local,
	const git_oid *upstream);

/**
 * Return the count of elements in the list.
 */
GIT_EXTERN(int) git_commit_list_count(git_commit_list *list);

/**
 * Return the nth element of the list.
 */
GIT_EXTERN(const git_oid *) git_commit_list_get_byindex(
	git_commit_list *list,
	size_t pos);

/**
 * Free a git_commit_list
 */
GIT_EXTERN(int) git_commit_list_free(  git_commit_list *list);