rebase: use staeobj to check whether interrupted rebase exists

This cleans up the code a bit.

Differential Revision: https://phab.mercurial-scm.org/D3875

hgext/rebase.py

@@ -250,7 +250,9 b' class rebaseruntime(object):'
         state = {}
         destmap = {}
 
-        try:
+        if not self.stateobj.exists():
+            cmdutil.wrongtooltocontinue(repo, _('rebase'))
+        else:
             f = repo.vfs("rebasestate")
             for i, l in enumerate(f.read().splitlines()):
                 if i == 0:
@@ -290,11 +292,6 b' class rebaseruntime(object):'
                     else:
                         state[oldrev] = repo[newrev].rev()
 
-        except IOError as err:
-            if err.errno != errno.ENOENT:
-                raise
-            cmdutil.wrongtooltocontinue(repo, _('rebase'))
-
         if data['keepbranches'] is None:
             raise error.Abort(_('.hg/rebasestate is incomplete'))