when: on_change (from fork point)

Description

I suggested this in gitlab-ce#23455 but it is really a separate feature. Often the line of mono-repo or many-repo is drawn at separate lifecycles. A project comprised of independent (but exclusive) microservices could benefit from a mono-repo but independent microservices need independent lifecycles (but not neccessarily different pipelines.

Proposal

We have "on_success, on_failure, always or manual options for when. I propose an option of "on_change" that will only execute if files in its context have changes. I started a Bash wrapper, lolaus, to do this but then the job still has to start to evaluate the wrapper and it would just be cleaner/better to have it built in.

build:
  image: dnd
  stage: build
  context: microservices-1/
  script:
    - docker build  .
  when: on_change

on_change can be something like, git merge-base --fork-point master $(git symbolic-ref --short HEAD)