Merge branch 'nd/commit-util-to-slab'

The in-core "commit" object had an all-purpose "void *util" field,
which was tricky to use especially in library-ish part of the
code.  All of the existing uses of the field has been migrated to a
more dedicated "commit-slab" mechanism and the field is eliminated.

* nd/commit-util-to-slab:
  commit.h: delete 'util' field in struct commit
  merge: use commit-slab in merge remote desc instead of commit->util
  log: use commit-slab in prepare_bases() instead of commit->util
  show-branch: note about its object flags usage
  show-branch: use commit-slab for commit-name instead of commit->util
  name-rev: use commit-slab for rev-name instead of commit->util
  bisect.c: use commit-slab for commit weight instead of commit->util
  revision.c: use commit-slab for show_source
  sequencer.c: use commit-slab to associate todo items to commits
  sequencer.c: use commit-slab to mark seen commits
  shallow.c: use commit-slab for commit depth instead of commit->util
  describe: use commit-slab for commit names instead of commit->util
  blame: use commit-slab for blame suspects instead of commit->util
  commit-slab: support shared commit-slab
  commit-slab.h: code split

commit.c

@@ -1605,13 +1605,21 @@ out:
 	return result;
 }
 
+define_commit_slab(merge_desc_slab, struct merge_remote_desc *);
+static struct merge_desc_slab merge_desc_slab = COMMIT_SLAB_INIT(1, merge_desc_slab);
+
+struct merge_remote_desc *merge_remote_util(struct commit *commit)
+{
+	return *merge_desc_slab_at(&merge_desc_slab, commit);
+}
+
 void set_merge_remote_desc(struct commit *commit,
 			   const char *name, struct object *obj)
 {
 	struct merge_remote_desc *desc;
 	FLEX_ALLOC_STR(desc, name, name);
 	desc->obj = obj;
-	commit->util = desc;
+	*merge_desc_slab_at(&merge_desc_slab, commit) = desc;
 }
 
 struct commit *get_merge_parent(const char *name)
@@ -1623,7 +1631,7 @@ struct commit *get_merge_parent(const char *name)
 		return NULL;
 	obj = parse_object(&oid);
 	commit = (struct commit *)peel_to_type(name, 0, obj, OBJ_COMMIT);
-	if (commit && !commit->util)
+	if (commit && !merge_remote_util(commit))
 		set_merge_remote_desc(commit, name, obj);
 	return commit;
 }