commit: Don't require a subject

In practice, a lot of subjects are empty, because the commit date
and branch are sufficient identification. For example, rpm-ostree
does not use subjects. It also doesn't use the command-line ostree
commit tool, so this was not a problem there, but this makes the
behavior consistent.

Also adds a test that empty subjects and omitting the subject
are equivalent. The --timestamp is so that the commits do not
have different timestamps.

Closes: #305
Approved by: cgwalters

src/ostree/ot-builtin-commit.c

@@ -437,13 +437,6 @@ ostree_builtin_commit (int argc, char **argv, GCancellable *cancellable, GError
         goto out;
     }
 
-  if (!opt_subject)
-    {
-      g_set_error_literal (error, G_IO_ERROR, G_IO_ERROR_FAILED,
-                           "A subject must be specified with --subject");
-      goto out;
-    }
-
   if (!ostree_repo_prepare_transaction (repo, NULL, cancellable, error))
     goto out;