Merge branch 'ds/ahead-behind'

"git for-each-ref" learns '%(ahead-behind:<base>)' that computes the
distances from a single reference point in the history with bunch
of commits in bulk.

* ds/ahead-behind:
  commit-reach: add tips_reachable_from_bases()
  for-each-ref: add ahead-behind format atom
  commit-reach: implement ahead_behind() logic
  commit-graph: introduce `ensure_generations_valid()`
  commit-graph: return generation from memory
  commit-graph: simplify compute_generation_numbers()
  commit-graph: refactor compute_topological_levels()
  for-each-ref: explicitly test no matches
  for-each-ref: add --stdin option

commit-graph.h

@@ -189,4 +189,12 @@ struct commit_graph_data {
  */
 timestamp_t commit_graph_generation(const struct commit *);
 uint32_t commit_graph_position(const struct commit *);
+
+/*
+ * After this method, all commits reachable from those in the given
+ * list will have non-zero, non-infinite generation numbers.
+ */
+void ensure_generations_valid(struct repository *r,
+			      struct commit **commits, size_t nr);
+
 #endif