Remember detected content type id in CompareConfiguration with the
CONTENT_TYPE key. This value can be picked up as a hint by
TextMergeViewer implementations (like GenericEditorMergeViewer) that
need to know which content type was detected by compare editor.

Most TextMergeViewer implementations don't need that hint because they
aren't generic and usually always know which content type they will have
- not so for GenericEditorMergeViewer which is generic.

The hint 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 always derived from the input (like data from git revisions
that don't have regular files or buffers associated to the viewer).

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