Previously, PolyGerrit relied on the default value of the loading flag
in the diff view to show/hide the appropriate diff panels (or loading
message). Because of this, stale information is displayed when a user
navigates between different files within the same diff view.
With this change, the `_loading` flag is set before the major API calls
are made, and the loading message is made more obvious.
Bug: Issue 7381
Change-Id: Id70d981a2c72fef9f44a32c5463dc43c3ce41cf6
29b2ab4a69