hgweb: walk the graph through the changelog

This is necessary to enforce filtering. The result is a bit buggy (may provide
less changeset than expected, but it will stop crashing on filtered revision
access.

Note that changelog.revs can not represents empty iteration like xrange did. So
we have to explicitly prevent call when there is nothing to do.

mercurial/hgweb/webcommands.py

@@ -857,7 +857,10 b' def graph(web, req, tmpl):'
     downrev = max(0, rev - revcount)
     changenav = webutil.revnav(web.repo).gen(pos, revcount, count)
 
-    dag = graphmod.dagwalker(web.repo, range(start, end)[::-1])
+    tree = []
+    if start < end:
+        revs = list(web.repo.changelog.revs(end - 1, start))
+        dag = graphmod.dagwalker(web.repo, revs)
     tree = list(graphmod.colored(dag, web.repo))
 
     def getcolumns(tree):

tests/test-obsolete.t

@@ -684,15 +684,15 b' check changelog view'
   $ "$TESTDIR/get-with-headers.py" --headeronly localhost:$HGPORT 'shortlog/'
   200 Script output follows
 
-#check graph view
-#
-#  $ "$TESTDIR/get-with-headers.py" --headeronly localhost:$HGPORT 'graph'
-#  200 Script output follows
+check graph view
+
+  $ "$TESTDIR/get-with-headers.py" --headeronly localhost:$HGPORT 'graph'
+  200 Script output follows
+
 check filelog view
 
   $ "$TESTDIR/get-with-headers.py" --headeronly localhost:$HGPORT 'log/'`hg id --debug --id`/'babar'
   200 Script output follows
-
   $ kill `cat hg.pid`
 
 Checking _enable=False warning if obsolete marker exists