simplify:inline variale
[git] rename test class to match the production class name
[git] remove trivial test setup
[git] fix refs comparator for the case when refs have equal names
[log] Simplify & fix refs comparison API & implementation

* Change VcsRefManager.sort to getComparator.
* Define ContainerUtil.sorted to return new list instead of sorting
  in place, since this pattern is used a lot.
* Clarify javadocs to clearly indicate what does each compare result
  mean.
* Fix GraphColorManagerImpl#compare according to this meaning &
  simplify comparison delegation to VcsLogRefManagers.
[git tests] assume git version is supported