histedit: add inprogress method to state class

If a histedit is progress, the 'histedit-state' file should exist. The patch
implements a convenience function to do check if a histedit is in progress.

This method will be use in next patch in the series.

hgext/histedit.py

@@ -312,6 +312,9 b' class histeditstate(object):'
     def clear(self):
         self.repo.vfs.unlink('histedit-state')
 
+    def inprogress(self):
+        return self.repo.vfs.exists('histedit-state')
+
 class histeditaction(object):
     def __init__(self, state, node):
         self.state = state