mergestate: add methods to queue files to remove, add or get

These are meant for use by custom merge drivers that might want to modify the
dirstate. Dirstate internal consistency rules require that all removes happen
before any adds -- this means that custom merge drivers shouldn't be modifying
the dirstate directly.

mercurial/merge.py

@@ -522,6 +522,24 b' class mergestate(object):'
         branchmerge = self._repo.dirstate.p2() != nullid
         recordupdates(self._repo, self.actions(), branchmerge)
 
+    def queueremove(self, f):
+        """queues a file to be removed from the dirstate
+
+        Meant for use by custom merge drivers."""
+        self._results[f] = 0, 'r'
+
+    def queueadd(self, f):
+        """queues a file to be added to the dirstate
+
+        Meant for use by custom merge drivers."""
+        self._results[f] = 0, 'a'
+
+    def queueget(self, f):
+        """queues a file to be marked modified in the dirstate
+
+        Meant for use by custom merge drivers."""
+        self._results[f] = 0, 'g'
+
 def _checkunknownfile(repo, wctx, mctx, f, f2=None):
     if f2 is None:
         f2 = f