commit: Don't require a subject

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