outgoing: adds some default value for argument

We are about to introduce a third option to create an outgoing object:
'missingroots'. This argument will be mutually exclusive with 'commonheads' so
we implement some default value handling in preparation.

This will also help use to make more use of outgoing creation around the code
base.

mercurial/discovery.py

@@ -76,10 +76,15 b' class outgoing(object):'
     The sets are computed on demand from the heads, unless provided upfront
     by discovery.'''
 
-    def __init__(self, repo, commonheads, missingheads):
+    def __init__(self, repo, commonheads=None, missingheads=None):
+        cl = repo.changelog
+        if not missingheads:
+            missingheads = cl.heads()
+        if not commonheads:
+            commonheads = [nullid]
         self.commonheads = commonheads
         self.missingheads = missingheads
-        self._revlog = repo.changelog
+        self._revlog = cl
         self._common = None
         self._missing = None
         self.excluded = []