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>

1 files changed, 2 insertions, 1 deletions

diff --git a/Documentation/git-for-each-ref.txt b/Documentation/git-for-each-ref.txt
index 3a271bf8af..324ad2c739 100644
--- a/Documentation/git-for-each-ref.txt
+++ b/Documentation/git-for-each-ref.txt
@@ -150,7 +150,8 @@ The complete message in a commit and tag object is `contents`.
 Its first line is `contents:subject`, where subject is the concatenation
 of all lines of the commit message up to the first blank line.  The next
 line is 'contents:body', where body is all of the lines after the first
-blank line.  Finally, the optional GPG signature is `contents:signature`.
+blank line.  The optional GPG signature is `contents:signature`.  The
+first `N` lines of the message is obtained using `contents:lines=N`.
 
 For sorting purposes, fields with numeric values sort in numeric
 order (`objectsize`, `authordate`, `committerdate`, `taggerdate`).