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

t/t4202-log.sh

@@ -887,4 +887,8 @@ test_expect_success GPG 'log --graph --show-signature for merged tag' '
 	grep "^| | gpg: Good signature" actual
 '
 
+test_expect_success 'log --graph --no-walk is forbidden' '
+	test_must_fail git log --graph --no-walk
+'
+
 test_done