Force PR creation if the commits are made by the script

Use case: our scripts already make the commits like we want them (GPG signed), but if no change is detected, git-xargs will skip.

Somehow similar to: https://github.com/gruntwork-io/git-xargs/issues/39

**Describe the solution you'd like**
a flag to force the push and PR creation

**Describe alternatives you've considered**
Modifying the commits created by git-xargs after the fact

**Additional context**
This allows GPG signatures, like proposed in https://github.com/gruntwork-io/git-xargs/pull/140, as well as chain of commits if an more understandable git history is desired.