New tests for merging through Team (GitSubscriberMergeContext).

IMergeContext offers API for clients to merge files or models through
pure Team APIs, without resorting to repository provider dependencies.
This adds tests for these actions.

Change-Id: Ia140f490d8a0831bc9fe6106d38b7078e3e6d93e
Signed-off-by: Laurent Goubet <laurent.goubet@obeo.fr>
Merge "New tests for merging through Team (GitSubscriberMergeContext)."