Add Summary & Description fields to commit message

[joewiz]

Git GUI clients like GitHub Desktop and Tower provide two fields for commit messages - a one-line Summary field (which invites brief, ~50 character summaries) and a multiline Description field (which invites longer descriptions if needed).

A screenshot showing this in GitHub Desktop - before text has been entered:

After text has been entered:

I imagine that since this oXygen Git UI is designed to fit inside a sidebar pane, you're working with constrained space. If #23 is adopted, though, it could potentially reclaim a lot of space, some of which could go to this additional field.