[refactoring] pull up the method to remove common fragments into the base class
add function to remove unmodified elements from target of link detection
implement a function to set a lower threshold for fragments to be registered into db
add a column to represent whether a file was deleted at the end revision
add a column to represent whether the owner file was deleted at the end revision
add a column to represent whether a file was added at the start revision
add a column to represent whether the owner file was added at the start revision
implement a special treat for classes
remove common code between Single and Multiple
update library files
add a class to represent concretized repositories
create a class to concretize repositoriesA
add repository manager in DataManagerManager
add repository manager in DBDataManagerManager
change RevisionInfo and its concretizer to fit the new scheme
create a class for combined revisions
update the reference
create the concretizer for combined revisions
add data managers for combined revisions
change the type of revisions from RevisionInfo to CombinedRevisoinInfo on CloneGenealogyInfo