pull: make discovery phase extensible

We apply the same approach as for push and make the discovery extensible. There
is only one user in core right now, but we already know we'll need something
smarter for obsmarkers. In fact the evolve extension could use this to cleanly
extend discovery.

The main motivation for this change is consistency between push and pull.

mercurial/exchange.py

@@ -868,7 +868,38 b' def pull(repo, remote, heads=None, force'
 
     return pullop
 
+# list of steps to perform discovery before pull
+pulldiscoveryorder = []
+
+# Mapping between step name and function
+#
+# This exists to help extensions wrap steps if necessary
+pulldiscoverymapping = {}
+
+def pulldiscovery(stepname):
+    """decorator for function performing discovery before pull
+
+    The function is added to the step -> function mapping and appended to the
+    list of steps.  Beware that decorated function will be added in order (this
+    may matter).
+
+    You can only use this decorator for a new step, if you want to wrap a step
+    from an extension, change the pulldiscovery dictionary directly."""
+    def dec(func):
+        assert stepname not in pulldiscoverymapping
+        pulldiscoverymapping[stepname] = func
+        pulldiscoveryorder.append(stepname)
+        return func
+    return dec
+
 def _pulldiscovery(pullop):
+    """Run all discovery steps"""
+    for stepname in pulldiscoveryorder:
+        step = pulldiscoverymapping[stepname]
+        step(pullop)
+
+@pulldiscovery('changegroup')
+def _pulldiscoverychangegroup(pullop):
     """discovery phase for the pull
 
     Current handle changeset discovery only, will change handle all discovery