commit/revisions: bookkeeping before refactoring

commit/revisions: bookkeeping before refactoring

There are a few things that need to move around a little before
making a big refactoring in the topo-order logic:

1. We need access to record_author_date() and
   compare_commits_by_author_date() in revision.c. These are used
   currently by sort_in_topological_order() in commit.c.

2. Moving these methods to commit.h requires adding an author_date_slab
   declaration to commit.h. Consumers will need their own implementation.

3. The add_parents_to_list() method in revision.c performs logic
   around the UNINTERESTING flag and other special cases depending
   on the struct rev_info. Allow this method to ignore a NULL 'list'
   parameter, as we will not be populating the list for our walk.
   Also rename the method to the slightly more generic name
   process_parents() to make clear that this method does more than
   add to a list (and no list is required anymore).

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>