Merge branch 'ds/reachable'

The code for computing history reachability has been shuffled,
obtained a bunch of new tests to cover them, and then being
improved.

* ds/reachable:
  commit-reach: correct accidental #include of C file
  commit-reach: use can_all_from_reach
  commit-reach: make can_all_from_reach... linear
  commit-reach: replace ref_newer logic
  test-reach: test commit_contains
  test-reach: test can_all_from_reach_with_flags
  test-reach: test reduce_heads
  test-reach: test get_merge_bases_many
  test-reach: test is_descendant_of
  test-reach: test in_merge_bases
  test-reach: create new test tool for ref_newer
  commit-reach: move can_all_from_reach_with_flags
  upload-pack: generalize commit date cutoff
  upload-pack: refactor ok_to_give_up()
  upload-pack: make reachable() more generic
  commit-reach: move commit_contains from ref-filter
  commit-reach: move ref_newer from remote.c
  commit.h: remove method declarations
  commit-reach: move walk methods from commit.c

t/helper/test-tool.c

@@ -27,6 +27,7 @@ static struct test_cmd cmds[] = {
 	{ "online-cpus", cmd__online_cpus },
 	{ "path-utils", cmd__path_utils },
 	{ "prio-queue", cmd__prio_queue },
+	{ "reach", cmd__reach },
 	{ "read-cache", cmd__read_cache },
 	{ "read-midx", cmd__read_midx },
 	{ "ref-store", cmd__ref_store },