revisions API: have release_revisions() release "topo_walk_info"

Refactor the existing reset_topo_walk() into a thin wrapper for a
release_revisions_topo_walk_info() + resetting the member to "NULL",
and call release_revisions_topo_walk_info() from release_revisions().

This fixes memory leaks that have been with us ever since
"topo_walk_info" was added to revision.[ch] in
f0d9cc4196 (revision.c: begin refactoring --topo-order logic,
2018-11-01).

Due to various other leaks this makes no tests pass in their entirety,
but e.g. before this running this on git.git:

    ./git -P log --pretty=tformat:"%P   %H | %s" --parents --full-history --topo-order -3 -- README.md

Would report under SANITIZE=leak:

    SUMMARY: LeakSanitizer: 531064 byte(s) leaked in 6 allocation(s).

Now we'll free all of that memory.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -2943,6 +2943,8 @@ static void release_revisions_mailmap(struct string_list *mailmap)
 	free(mailmap);
 }
 
+static void release_revisions_topo_walk_info(struct topo_walk_info *info);
+
 void release_revisions(struct rev_info *revs)
 {
 	free_commit_list(revs->commits);
@@ -2956,6 +2958,7 @@ void release_revisions(struct rev_info *revs)
 	free_grep_patterns(&revs->grep_filter);
 	diff_free(&revs->pruning);
 	reflog_walk_info_release(revs->reflog_info);
+	release_revisions_topo_walk_info(revs->topo_walk_info);
 }
 
 static void add_child(struct rev_info *revs, struct commit *parent, struct commit *child)
@@ -3468,17 +3471,22 @@ static void compute_indegrees_to_depth(struct rev_info *revs,
 		indegree_walk_step(revs);
 }
 
-static void reset_topo_walk(struct rev_info *revs)
+static void release_revisions_topo_walk_info(struct topo_walk_info *info)
 {
-	struct topo_walk_info *info = revs->topo_walk_info;
-
+	if (!info)
+		return;
 	clear_prio_queue(&info->explore_queue);
 	clear_prio_queue(&info->indegree_queue);
 	clear_prio_queue(&info->topo_queue);
 	clear_indegree_slab(&info->indegree);
 	clear_author_date_slab(&info->author_date);
+	free(info);
+}
 
-	FREE_AND_NULL(revs->topo_walk_info);
+static void reset_topo_walk(struct rev_info *revs)
+{
+	release_revisions_topo_walk_info(revs->topo_walk_info);
+	revs->topo_walk_info = NULL;
 }
 
 static void init_topo_walk(struct rev_info *revs)