ostree: Support for using EDITOR to fill commit subject/body

Behave similar to git when 'ostree commit' is run without
a --subject or --body. Bring up an editor. The first line becomes
the subject and following lines become the --body after an optional
blank line.

Use similar logic to git in determining EDITOR

https://bugzilla.gnome.org/show_bug.cgi?id=707063