Merge branch 'ds/rev-list-maximal-only-optim'

"git rev-list --maximal-only" has been optimized by borrowing the
logic used by "git show-branch --independent", which computes the
same kind of information much more efficiently.

* ds/rev-list-maximal-only-optim:
  rev-list: use reduce_heads() for --maximal-only
  p6011: add perf test for rev-list --maximal-only
  t6600: test --maximal-only and --independent