Use when keyword for pipeline conditions

[anbraten]

Currently there is a single condition for whole pipeline files named branches. As woodpecker will probably support more conditions in the future like #86 and to match the way conditions for pipeline-steps are working it would be nice to move the branch condition below the when keyword.

pipeline:
  build:
    image: golang
    commands:
      - go build
      - go test
   when:
     branch: [master, develop]

-branches: [ master, develop ]
+when:
+  branch: [master, develop]

[6543]

yes we should unify the when filter for pipeline and steps ...