changegroup: refactor outgoing logic into a function

Extensions that add to bundle2 will want to know which commits are outgoing so
they can bundle data that is appropriate to those commits. This moves the logic
for figuring that out to a separate function so extensions can do the same
computation.

mercurial/changegroup.py

@@ -493,6 +493,25 b' def getlocalbundle(repo, source, outgoin'
     bundler = bundle10(repo, bundlecaps)
     return getsubset(repo, outgoing, bundler, source)
 
+def _computeoutgoing(repo, heads, common):
+    """Computes which revs are outgoing given a set of common
+    and a set of heads.
+
+    This is a separate function so extensions can have access to
+    the logic.
+
+    Returns a discovery.outgoing object.
+    """
+    cl = repo.changelog
+    if common:
+        hasnode = cl.hasnode
+        common = [n for n in common if hasnode(n)]
+    else:
+        common = [nullid]
+    if not heads:
+        heads = cl.heads()
+    return discovery.outgoing(cl, common, heads)
+
 def getbundle(repo, source, heads=None, common=None, bundlecaps=None):
     """Like changegroupsubset, but returns the set difference between the
     ancestors of heads and the ancestors common.
@@ -502,15 +521,7 b' def getbundle(repo, source, heads=None, '
     The nodes in common might not all be known locally due to the way the
     current discovery protocol works.
     """
-    cl = repo.changelog
-    if common:
-        hasnode = cl.hasnode
-        common = [n for n in common if hasnode(n)]
-    else:
-        common = [nullid]
-    if not heads:
-        heads = cl.heads()
-    outgoing = discovery.outgoing(cl, common, heads)
+    outgoing = _computeoutgoing(repo, heads, common)
     return getlocalbundle(repo, source, outgoing, bundlecaps=bundlecaps)
 
 def changegroup(repo, basenodes, source):