Detected content type id could be retrieved now from the
CompareConfiguration with the CONTENT_TYPE key (see
https://github.com/eclipse-platform/eclipse.platform/pull/1294).

Use this value (if available) as fallbackContentTypes in
ExtensionBasedTextViewerConfiguration.

The allows GenericEditorMergeViewer to know for which content type
was it actually created and so use that in cases where the content type
can't be derived from the input (like data from git revisions
that don't have regular files or buffers associated to the viewer).

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1747