`gitree -m` - output 'no modifications' message if there are no modifications

[aSemy]

When I run git status on a clean branch, I get a message saying there are no changes.

When I run gitree -m, there is no output. This is a bit confusing, because I'm not sure if the command ran successfully or not.

(By the way, really cool package! It really helps me)

[jpwilliams]

This is a great suggestion. Open to PRs for this before I have time to get around to it. :)