hgweb: let the web graph cope with low revisions/new repositories (issue1293)

mercurial/graphmod.py

@@ -27,6 +27,7 b' def graph(repo, start_rev, stop_rev):'
     """
 
     assert start_rev >= stop_rev
+    assert stop_rev >= 0
     curr_rev = start_rev
     revs = []
     cl = repo.changelog

mercurial/hgweb/webcommands.py

@@ -588,7 +588,7 b' def graph(web, req, tmpl):'
     count = len(web.repo)
     changenav = webutil.revnavgen(rev, maxchanges, count, web.repo.changectx)
 
-    tree = list(graphmod.graph(web.repo, rev, rev - revcount))
+    tree = list(graphmod.graph(web.repo, rev, downrev))
     canvasheight = (len(tree) + 1) * bg_height - 27;
 
     data = []