histedit: add histedit state class

Add an histeditstate class that is intended to hold the current
state.  This allows us encapsulate the state and avoids passing
around a tuple which is based on the serialization format. In
particular this will give actions more control over the state and
allow external sources to have more control of histedits behavior,
e.g. an external implementation of x/exec.

hgext/histedit.py

@@ -188,6 +188,25 b' editcomment = _("""# Edit history betwee'
 #
 """)
 
+class histeditstate(object):
+    def __init__(self, repo, parentctxnode=None, rules=None, keep=None,
+            topmost=None, replacements=None):
+        self.repo = repo
+        self.parentctxnode = parentctxnode
+        self.rules = rules
+        self.keep = keep
+        self.topmost = topmost
+        if replacements is None:
+            self.replacements = []
+        else:
+            self.replacements = replacements
+
+    def write(self):
+        fp = self.repo.vfs('histedit-state', 'w')
+        pickle.dump((self.parentctxnode, self.rules, self.keep,
+            self.topmost, self.replacements), fp)
+        fp.close()
+
 def commitfuncfor(repo, src):
     """Build a commit function for the replacement of <src>