ref-filter: make internal reachable-filter API more precise

The internal reachable-filter API is a bit loose and imprecise; it
also bleeds unnecessarily into the public header. Tighten the API
by:

* renaming do_merge_filter() to reach_filter()

* separating parameters to explicitly identify what data is used
  by the function instead of passing an entire ref_filter_cbdata
  struct

* renaming and moving internal constants from header to source
  file

Signed-off-by: Aaron Lipman <alipman88@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -2231,19 +2231,18 @@ void ref_array_clear(struct ref_array *array)
 	}
 }
 
-static void do_merge_filter(struct ref_filter_cbdata *ref_cbdata, int reachable)
+#define EXCLUDE_REACHED 0
+#define INCLUDE_REACHED 1
+static void reach_filter(struct ref_array *array,
+			 struct commit_list *check_reachable,
+			 int include_reached)
 {
 	struct rev_info revs;
 	int i, old_nr;
-	struct ref_array *array = ref_cbdata->array;
 	struct commit **to_clear = xcalloc(sizeof(struct commit *), array->nr);
-	struct commit_list *rl;
+	struct commit_list *cr;
 
-	struct commit_list *check_reachable_list = reachable ?
-		ref_cbdata->filter->reachable_from :
-		ref_cbdata->filter->unreachable_from;
-
-	if (!check_reachable_list)
+	if (!check_reachable)
 		return;
 
 	repo_init_revisions(the_repository, &revs, NULL);
@@ -2254,8 +2253,8 @@ static void do_merge_filter(struct ref_filter_cbdata *ref_cbdata, int reachable)
 		to_clear[i] = item->commit;
 	}
 
-	for (rl = check_reachable_list; rl; rl = rl->next) {
-		struct commit *merge_commit = rl->item;
+	for (cr = check_reachable; cr; cr = cr->next) {
+		struct commit *merge_commit = cr->item;
 		merge_commit->object.flags |= UNINTERESTING;
 		add_pending_object(&revs, &merge_commit->object, "");
 	}
@@ -2273,16 +2272,16 @@ static void do_merge_filter(struct ref_filter_cbdata *ref_cbdata, int reachable)
 
 		int is_merged = !!(commit->object.flags & UNINTERESTING);
 
-		if (is_merged == reachable)
+		if (is_merged == include_reached)
 			array->items[array->nr++] = array->items[i];
 		else
 			free_array_item(item);
 	}
 
 	clear_commit_marks_many(old_nr, to_clear, ALL_REV_FLAGS);
 
-	while (check_reachable_list) {
-		struct commit *merge_commit = pop_commit(&check_reachable_list);
+	while (check_reachable) {
+		struct commit *merge_commit = pop_commit(&check_reachable);
 		clear_commit_marks(merge_commit, ALL_REV_FLAGS);
 	}
 
@@ -2337,8 +2336,8 @@ int filter_refs(struct ref_array *array, struct ref_filter *filter, unsigned int
 	clear_contains_cache(&ref_cbdata.no_contains_cache);
 
 	/*  Filters that need revision walking */
-	do_merge_filter(&ref_cbdata, DO_MERGE_FILTER_REACHABLE);
-	do_merge_filter(&ref_cbdata, DO_MERGE_FILTER_UNREACHABLE);
+	reach_filter(array, filter->reachable_from, INCLUDE_REACHED);
+	reach_filter(array, filter->unreachable_from, EXCLUDE_REACHED);
 
 	return ret;
 }

ref-filter.h

@@ -23,9 +23,6 @@
 #define FILTER_REFS_DETACHED_HEAD  0x0020
 #define FILTER_REFS_KIND_MASK      (FILTER_REFS_ALL | FILTER_REFS_DETACHED_HEAD)
 
-#define DO_MERGE_FILTER_UNREACHABLE 0
-#define DO_MERGE_FILTER_REACHABLE   1
-
 struct atom_value;
 
 struct ref_sorting {