line is 'contents:body', where body is all of the lines after the first
blank line. The optional GPG signature is `contents:signature`. The
first `N` lines of the message is obtained using `contents:lines=N`.
+Additionally, the trailers as interpreted by linkgit:git-interpret-trailers[1]
+are obtained as 'contents:trailers'.
For sorting purposes, fields with numeric values sort in numeric order
(`objectsize`, `authordate`, `committerdate`, `creatordate`, `taggerdate`).