merge: record the "other" node in merge state

We need to record the merge we were merging with. This solve multiple
bug with resolve when dropping the second parent after a merge. This
happen a lot when doing special merge (overriding the ancestor).
Backout, shelve, rebase, etc. can takes advantage of it.

This changeset just add the information in the merge state. We'll use it in the
resolve process in a later changeset.

mercurial/merge.py

@@ -36,6 +36,7 b' class mergestate(object):'
     Currently known record:
 
     L: the node of the "local" part of the merge (hexified version)
+    O: the node of the "other" part of the merge (hexified version)
     F: a file to be merged entry
     '''
     statepathv1 = "merge/state"
@@ -44,10 +45,11 b' class mergestate(object):'
         self._repo = repo
         self._dirty = False
         self._read()
-    def reset(self, node=None):
+    def reset(self, node=None, other=None):
         self._state = {}
         if node:
             self._local = node
+            self._other = other
         shutil.rmtree(self._repo.join("merge"), True)
         self._dirty = False
     def _read(self):
@@ -56,6 +58,8 b' class mergestate(object):'
         for rtype, record in records:
             if rtype == 'L':
                 self._local = bin(record)
+            elif rtype == 'O':
+                self._other = bin(record)
             elif rtype == "F":
                 bits = record.split("\0")
                 self._state[bits[0]] = bits[1:]
@@ -111,6 +115,7 b' class mergestate(object):'
         if self._dirty:
             records = []
             records.append(("L", hex(self._local)))
+            records.append(("O", hex(self._other)))
             for d, v in self._state.iteritems():
                 records.append(("F", "\0".join([d] + v)))
             self._writerecords(records)
@@ -529,7 +534,7 b' def applyupdates(repo, actions, wctx, mc'
 
     updated, merged, removed, unresolved = 0, 0, 0, 0
     ms = mergestate(repo)
-    ms.reset(wctx.p1().node())
+    ms.reset(wctx.p1().node(), mctx.node())
     moves = []
     actions.sort(key=actionkey)