Feature Request: short-circuit CI pipelines based on files changed since the previous passed pipeline

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Currently we waste a lot of resources running ruby unit tests when unrelated changes have been made (CSS, JS, documentation changes, etc). I believe we can improve on this by exposing more context-based configuration options within .gitlab-ci.yml.

What if we could do the following:

1. Pipeline is created.
2. Find SHA for the most recent ancestor with a passing build.
3. Generate a diff between the pipeline's commit SHA and the most recent successful ancestor.
4. Run configured rules to determine which tests to run.

Some example rules:

IF no file matching /**/.eslintrc or /**/*.js has changed, THEN skip the eslint build step.
IF no file matching /**/*.rb has been changed, THEN skip rspec unit tests. (feature tests still run)
IF no file matching /**/.rubocop.yml or /**/*.rb has been changed, THEN skip rubocop linter

This could go into the .gitlab-ci.yml like so:

# ...

lint:javascript:
  <<: *dedicated-runner
  cache:
    paths:
      - node_modules/
  stage: test
  if_changed:
    - "/**/.eslintrc"
    - "/**/*.js"
  image: "node:7.1"
  before_script:
    - npm install
  script:
    - npm --silent run eslint

# ...

Practical applications

Save on build time for certain types of merge requests:

Table of builds to run (using GitLab CE's pipeline config as an example)

| | rubocop | rspec | rake db:migrate | rake haml_lint | rake scss_lint | lint:javascript | teaspoon | lint-doc | |-----|-----|-----|-----|-----|-----|-----|-----|-----|-----| | Markdown File Change | no | no | no | no | no | no | no | yes | | JavaScript Changes | no | feature tests only | no | no | no | yes | yes | no | | SCSS Changes | no | feature tests only | no | no | yes | no | yes | no | | Haml Changes | yes | feature tests only | no | yes | no | no | yes | no | | Ruby Changes (not in /db/*or /config/*) | yes | yes | no | no | no | no | yes | no | | Ruby Changes (/db/migrate/foo.rb) | yes | yes | yes | no | no | no | yes | no |