Support git commit body

[ghost]

Is your feature request related to a problem? Please describe.
When doing a commit in gitui, I often need to use the external editor because gitui does not allow for git commit body text.

Describe the solution you'd like
To be able to make a new line in the git commit window to add a git commit body and not only the subject as it is now.

Describe alternatives you've considered
Right now I am forced to use the external editor, which kind of defeat the purpose of using gitui for such a simple thing as a commit.

[extrawurst]

duplicate of #1171