graphmod: don't try to visit nullrev (issue3772)

mercurial/graphmod.py

@@ -181,7 +181,7 b' def asciiedges(type, char, lines, seen, '
     ncols = len(seen)
     nextseen = seen[:]
     nextseen[nodeidx:nodeidx + 1] = newparents
-    edges = [(nodeidx, nextseen.index(p)) for p in knownparents]
+    edges = [(nodeidx, nextseen.index(p)) for p in knownparents if p != nullrev]
 
     while len(newparents) > 2:
         # ascii() only knows how to add or remove a single column between two

tests/test-glog.t

@@ -2109,4 +2109,12 b' The almost-empty template should do some'
   o
   
 
+issue3772
+
+  $ hg glog -r :null
+  o  changeset:   -1:000000000000
+     user:
+     date:        Thu Jan 01 00:00:00 1970 +0000
+  
+
   $ cd ..