Provide commitId for getAllElements

[wobrschalek]

Problem:
For supporting a gitflow model with MMS (e.g. with LemonTree) in an operational safe way, we need to be sure that we write (createOrUpdateElements) only to a branch that didn't change since our pull (getAllElements).
The only way to ensure this, is by using the commitId, but currently there is no way to accomplish that with a single call to the API.

proposed solution
The endpoint getAllElements (/projects/{projectId}/refs/{refId}/elements) returns also the commitId.