ref-filter: implement '--contains' option

'tag -l' and 'branch -l' have two different ways of finding
out if a certain ref contains a commit. Implement both these
methods in ref-filter and give the caller of ref-filter API
the option to pick which implementation to be used.

'branch -l' uses 'is_descendant_of()' from commit.c which is
left as the default implementation to be used.

'tag -l' uses a more specific algorithm since ffc4b80. This
implementation is used whenever the 'with_commit_tag_algo' bit
is set in 'struct ref_filter'.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -86,6 +86,11 @@ static int in_commit_list(const struct commit_list *want, struct commit *c)
 	return 0;
 }
 
+/*
+ * The entire code segment for supporting the --contains option has been
+ * copied over to ref-filter.{c,h}. This will be deleted evetually when
+ * we port tag.c to use ref-filter APIs.
+ */
 enum contains_result {
 	CONTAINS_UNKNOWN = -1,
 	CONTAINS_NO = 0,