histedit: use self.stateobj to check whether interrupted histedit exists

self.stateobj is an instance of state.cmdstate() class which has an .exists()
function which is used to check whether there exists an interrupted statefile or
not.

Differential Revision: https://phab.mercurial-scm.org/D3865

hgext/histedit.py

@@ -183,7 +183,6 b' unexpectedly::'
 
 from __future__ import absolute_import
 
-import errno
 import os
 
 from mercurial.i18n import _
@@ -313,14 +312,10 b' class histeditstate(object):'
 
     def read(self):
         """Load histedit state from disk and set fields appropriately."""
-        try:
-            state = self.repo.vfs.read('histedit-state')
-        except IOError as err:
-            if err.errno != errno.ENOENT:
-                raise
+        if not self.stateobj.exists():
             cmdutil.wrongtooltocontinue(self.repo, _('histedit'))
 
-        data = self._read(state)
+        data = self._read()
 
         self.parentctxnode = data['parentctxnode']
         actions = parserules(data['rules'], self)
@@ -330,7 +325,8 b' class histeditstate(object):'
         self.replacements = data['replacements']
         self.backupfile = data['backupfile']
 
-    def _read(self, fp):
+    def _read(self):
+        fp = self.repo.vfs.read('histedit-state')
         if fp.startswith('v1\n'):
             data = self._load()
             parentctxnode, rules, keep, topmost, replacements, backupfile = data