Extract hover manager of CommitGraphTable into own file

This is a refactoring because the class will have to grow in the next
change.

Change-Id: I04022b27c8beddaa34acdeceb2982c70f8344f1f
Signed-off-by: Robin Stocker <robin@nibor.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>