commit-reach: add general graph traversal find_reachable()

Add find_reachable(), a generalization of the memoized DFS in
can_all_from_reach_with_flag().

The benefits are two-fold:
1. make the code more uniform - fewer core graph traversals
   to maintain and reason about.
2. optimization for ref-filter --contains

Most converted callers get equal or slightly better performance,
since this standardizes on the generally "best" implementation.

The big win is ref-filter --contains/--no-contains.
Instead of calling commit_contains() for each ref, we batch it
into batched calls to find_reachable(), changing the time complexity
from O(N * D) to O(D + N) where D is the reachable graph depth and
N is the number of refs.

The removed contains_tag_algo used the same memoized DFS approach
with a commit slab instead of flag bits; the batched
find_reachable_list produces identical results, as confirmed by
the existing test suites (t7004-tag, t3201-branch-contains,
t6302-for-each-ref-filter, t6600-test-reach: 370 tests total).

Signed-off-by: Kristofer Karlsson <krka@spotify.com>

Author: spkrka

builtin/tag.c

@@ -71,7 +71,6 @@ static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting,
 
 	if (verify_ref_format(format))
 		die(_("unable to parse format string"));
-	filter->with_commit_tag_algo = 1;
 	filter_and_format_refs(filter, FILTER_REFS_TAGS, sorting, format);
 
 	free(to_free);