revset: have all() filter out null revision

I'm not sure if "all()" should filter out "null", but "all()" is stated as
'the same as "0:tip"' (except that it doesn't reorder the subset, I think.)

This patch is intended to avoid exposing a fullreposet to graphmod.dagwalker(),
which would result in strange drawing in future version:

  |
  o  changeset:   0:f8035bb17114
  |  user:        test
  |  date:        Thu Jan 01 00:00:00 1970 +0000
  |  summary:     add a

caused by:

    parents = sorted(set([p.rev() for p in ctx.parents()
                          if p.rev() in revs]))

We cannot add "and p.rev() != nullrev" here because revs may actually include
"null" revision.

mercurial/revset.py

@@ -994,7 +994,7 b' def getall(repo, subset, x):'
     """
     # i18n: "all" is a keyword
     getargs(x, 0, 0, _("all takes no arguments"))
-    return subset
+    return subset & spanset(repo)  # drop "null" if any
 
 def grep(repo, subset, x):
     """``grep(regex)``

tests/test-glog.t

@@ -2365,4 +2365,14 b' issue3772'
      date:        Thu Jan 01 00:00:00 1970 +0000
   
 
+should not draw line down to null due to the magic of fullreposet
+
+  $ hg log -G -r 'all()' | tail -6
+  |
+  o  changeset:   0:f8035bb17114
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     add a
+  
+
   $ cd ..

tests/test-revset.t

@@ -480,6 +480,9 b' Test explicit numeric revision'
 Test null revision
   $ log 'ancestors(null)'
   -1
+  $ log 'tip:null and all()' | tail -2
+  1
+  0
 
   $ log 'outgoing()'
   8