[log] move structure filter implementation to vcs-log-impl
[vcs] VcsStructureChooser: remove dependency on the AbstractVcs.
[log] IDEA-116022 Implement Structure filter for VCS Log & Git

Mercurial will follow later: IDEA-116241
[log] Fix hidden graph because of the date filter existance

If no date filter is chosen, return null as specified by getFilter().