merge.mergestate: add a way to get the other side of the merge

It's surprising there was no API at all for this until now. In any case this
will be needed from custom merge drivers.

mercurial/merge.py

@@ -84,6 +84,8 b' class mergestate(object):'
         self._state = {}
         self._local = None
         self._other = None
+        if 'otherctx' in vars(self):
+            del self.otherctx
         if node:
             self._local = node
             self._other = other
@@ -101,6 +103,8 b' class mergestate(object):'
         self._local = None
         self._other = None
         self._mdstate = 'u'
+        if 'otherctx' in vars(self):
+            del self.otherctx
         records = self._readrecords()
         for rtype, record in records:
             if rtype == 'L':
@@ -233,6 +237,9 b' class mergestate(object):'
     @util.propertycache
     def mergedriver(self):
         return self._repo.ui.config('experimental', 'mergedriver')
+    @util.propertycache
+    def otherctx(self):
+        return self._repo[self._other]
 
     def active(self):
         """Whether mergestate is active.