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>

t/t6302-for-each-ref-filter.sh

@@ -167,4 +167,56 @@ test_expect_success 'nested alignment with quote formatting' "
 	test_cmp expect actual
 "
 
+test_expect_success 'check `%(contents:lines=1)`' '
+	cat >expect <<-\EOF &&
+	master |three
+	side |four
+	odd/spot |three
+	double-tag |Annonated doubly
+	four |four
+	one |one
+	signed-tag |A signed tag message
+	three |three
+	two |two
+	EOF
+	git for-each-ref --format="%(refname:short) |%(contents:lines=1)" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'check `%(contents:lines=0)`' '
+	cat >expect <<-\EOF &&
+	master |
+	side |
+	odd/spot |
+	double-tag |
+	four |
+	one |
+	signed-tag |
+	three |
+	two |
+	EOF
+	git for-each-ref --format="%(refname:short) |%(contents:lines=0)" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'check `%(contents:lines=99999)`' '
+	cat >expect <<-\EOF &&
+	master |three
+	side |four
+	odd/spot |three
+	double-tag |Annonated doubly
+	four |four
+	one |one
+	signed-tag |A signed tag message
+	three |three
+	two |two
+	EOF
+	git for-each-ref --format="%(refname:short) |%(contents:lines=99999)" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '`%(contents:lines=-1)` should fail' '
+	test_must_fail git for-each-ref --format="%(refname:short) |%(contents:lines=-1)"
+'
+
 test_done