Add Signed-off-by line to git.UserCommit
Add Signed-off-by line to git.UserCommit

This patch adds the `Signed-off-by: User Name <user@email.com>` line to
a `git.UserCommit`. The surrounding documentation and error messages have
been improved as well.

Signed-off-by: Sascha Grunert <70ab469ddb2ac3e35f32ed7c2fd1cca514b2e879@suse.com>

kubernetes/release,kubernetes/release