create an abstract class for all registerers
create a class to register combined revisions
[bug fix] a bug in createPreparedStatementsQueue was fixed
create a class to register combined commits
[bug fix] fix the mismatch of db scheme and class
make FileRegisterer to fit the new scheme
change CloneSetRegisterer to fit the new schene
change CodeFragmentLinkRegisterer to fit the new scheme
change CloneSetLinkRegisterer to fit the new scheme
change CloneSetGenealogyRegisterer to fit the new scheme
change CodeFragmentGenealogyRegisterer to fit the new scheme
[refactoring] rename
create an abstract class to retrieve all elements
change AbstractUniqueElementRetriever to inheritate AbstractElementRetriever
create a class to retrieve repositories
change RevisionRetriever to fit the new scheme
change CommitRetriever to fit the new scheme
create a class to retrieve combined revisions
create a class to retrieve combined commits
change FileRetriever to fit the new scheme