hgk: don't break on repositories with obsolete changesets

Check the existence of a changeset before adding it to the list
returned by debug-rev-list command.

hgext/hgk.py

@@ -204,9 +204,11 b' def revtree(ui, args, repo, full="tree",'
                     l[chunk - x:] = [0] * (chunk - x)
                     break
                 if full is not None:
+                    if (i + x) in repo:
                     l[x] = repo[i + x]
                     l[x].changeset() # force reading
                 else:
+                    if (i + x) in repo:
                     l[x] = 1
             for x in xrange(chunk - 1, -1, -1):
                 if l[x] != 0:
@@ -259,6 +261,8 b' def revtree(ui, args, repo, full="tree",'
     # walk the repository looking for commits that are in our
     # reachability graph
     for i, ctx in chlogwalk():
+        if i not in repo:
+            continue
         n = repo.changelog.node(i)
         mask = is_reachable(want_sha1, reachable, n)
         if mask: