Allow git commits without a build

Sometimes you just want to add things to the repository but don't trigger a build:

- Readme updates

- D8 configuration changes which already happened on production

- Changes to platform.sh unrelated files

As long as deployments cause downtime this is frustrating and hard to compensate.

All other CI / CD systems we use allow to skip builds by providing comments in the commit message like [SKIP-CI]. Implement something like this also on psh.