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`).
All other fields are used to sort in their byte-value order.
+There is also an option to sort by versions, this can be done by using
+the fieldname `version:refname` or its alias `v:refname`.
+
In any case, a field name that refers to a field inapplicable to
the object referred by the ref does not cause an error. It
returns an empty string instead.