Merge branch 'ps/shift-root-in-graph' into jch

Merge branch 'ps/shift-root-in-graph' into jch

"git log --graph" has been modified to visually distinguish
parentless "root" commits (and commits that become roots due to
history simplification) by indenting them, preventing them from
appearing falsely related to unrelated commits rendered immediately
above them.

* ps/shift-root-in-graph:
  graph: indent visual root in graph
  revision: add peek functions for lookahead
  lib-log-graph: move check_graph function