rebase: use repo.vfs.exists in 'hg summary' hook

rebase: use repo.vfs.exists in 'hg summary' hook

--- a/hgext/rebase.py	Wed Dec 28 15:48:17 2016 -0700
+++ b/hgext/rebase.py	Fri Jan 06 09:58:41 2017 +0100
@@ -1424,7 +1424,7 @@
     return obsoletenotrebased
 
 def summaryhook(ui, repo):
-    if not os.path.exists(repo.join('rebasestate')):
+    if not repo.vfs.exists('rebasestate'):
         return
     try:
         rbsrt = rebaseruntime(repo, ui, {})