Fix test_commit_msg_hook_success.

[uri-canva]

Test hooks are created with a sh shebang: https://github.com/gitpython-developers/GitPython/blob/master/git/test/test_index.py#L53

however in sh echo doesn't always support -n, and can print it out:

$ bash -c "echo -n hello"
hello # <- no newline here
$ sh -c "echo -n hello"
-n hello # <- newline here

Instead of echo -n use printf which works correctly in all shs.

[Byron]

Thanks a lot for your contribution :)!
Keep 'em coming!