add fetchInitialRevisions

[ghedamat]

this implements ember-cli-deploy/ember-cli-deploy#338

[ghedamat]

@lukemelia initially I merged revisionData from fetchInitialRevisions manually but this https://github.com/ember-cli/ember-cli-deploy/blob/master/lib/models/pipeline.js#L168-L172 should do that for us right?

[lukemelia]

@ghedamat While writing 0.6.x docs, I rethought this a little (see results at http://ember-cli.com/ember-cli-deploy/docs/v0.6.x/pipeline-hooks/)

fetchInitialRevisions -->  returns an object (or a promise resolving to
                           one) that has an `initialRevisions` property
                           whose value is an array of revision objects.[1]
                           i.e. `{ initialRevisions: [...] }`

and

            \- fetchRevisions  returns an object (or a promise resolving
             \                 to one) that has a `revisions` property
              \                whose value is an array of revision
               \               objects.[1] i.e. `{ revisions: [...] }`

[lukemelia]

This would indeed be a breaking change. We need to think about backwards compatibility and/or messaging.

[ghedamat]

@lukemelia I tend to agree, if we keep the properties separate we don't need to break any existing plugins and we can "reserve" the property names in the docs as you did

I'll update the PR and do the other ones later this week

[ghedamat]

@lukemelia should be good now and non breaking

[lukemelia]

test name is no longer correct

[ghedamat]

right, will fix soon