histedit: check presence of statefile before deleting it

When the histeditstate class instance has it's clear() method called, there is
nothing to check to see if the state file exists before deleting it. It may not
exist, which would create an exception. This patch allows clear to be called at
any time.

This will be needed for the following patch.

hgext/histedit.py

@@ -310,7 +310,8 b' class histeditstate(object):'
         return parentctxnode, rules, keep, topmost, replacements, backupfile
 
     def clear(self):
-        self.repo.vfs.unlink('histedit-state')
+        if self.inprogress():
+            self.repo.vfs.unlink('histedit-state')
 
     def inprogress(self):
         return self.repo.vfs.exists('histedit-state')