log: Display "(no subject)" for commits without a subject
log: Display "(no subject)" for commits without a subject

This resolves the ambiguity for a message with a body but no subject,
and also hints that OSTree is using two bytes to store the empty
commit subject/body (when instead they could be stored, or rather,
not stored, as part of the metadata)

Closes: #305
Approved by: cgwalters

gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree