Merge branch 'dj/log-graph-with-no-walk'

Merge branch 'dj/log-graph-with-no-walk'

"git log --graph --no-walk A B..." is a otcnflicting request that
asks nonsense; no-walk tells us show discrete points in the
history, while graph asks to draw connections between these
discrete points. Forbid the combination.

* dj/log-graph-with-no-walk:
  revision: forbid combining --graph and --no-walk