ref-filter: add support for %(contents:lines=X)

In 'tag.c' we can print N lines from the annotation of the tag using
the '-n<num>' option. Copy code from 'tag.c' to 'ref-filter' and
modify it to support appending of N lines from the annotation of tags
to the given strbuf.

Implement %(contents:lines=X) where X lines of the given object are
obtained.

While we're at it, remove unused "contents:<suboption>" atoms from
the `valid_atom` array.

Add documentation and test for the same.

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>

ref-filter.h

@@ -59,7 +59,8 @@ struct ref_filter {
 	struct commit *merge_commit;
 
 	unsigned int with_commit_tag_algo : 1;
-	unsigned int kind;
+	unsigned int kind,
+		lines;
 };
 
 struct ref_filter_cbdata {