@NotNull
  private Map<VcsRoot, Set<FilePath>> groupFilesByVcs(@Nullable Collection<? extends VirtualFile> from) {
    if (from == null) return Collections.emptyMap();
    return groupByVcs(() -> ContainerUtil.mapIterator(from.iterator(), file -> VcsUtil.getFilePath(file)));
  }
@NotNull
  private Map<VcsRoot, Set<FilePath>> groupFilesByVcs(@Nullable Collection<? extends VirtualFile> from)