destupdate: extract logic based on obsolescence marker in its own function

One of the main goal of having consolidated destination function is to allow
extension to play with this logic. We extract sub logic to make is wrapping more
practical.

mercurial/destutil.py

@@ -40,20 +40,12 b' def _destupdatevalidate(repo, rev, clean'
                     hint = _("merge or update --check to force update")
                     raise error.UpdateAbort(msg, hint=hint)
 
-def destupdate(repo, clean=False, check=False):
-    """destination for bare update operation
-
-    return (rev, movemark, activemark)
-
-    - rev: the revision to update to,
-    - movemark: node to move the active bookmark from
-                (cf bookmark.calculate update),
-    - activemark: a bookmark to activate at the end of the update.
-    """
+def _destupdateobs(repo, clean, check):
+    """decide of an update destination from obsolescence markers"""
     node = None
     wc = repo[None]
     p1 = wc.p1()
-    movemark = activemark = None
+    movemark = None
 
     if p1.obsolete() and not p1.children():
         # allow updating to successors
@@ -82,6 +74,23 b' def destupdate(repo, clean=False, check='
             node = repo.revs('max(%ln)', successors).first()
             if bookmarks.isactivewdirparent(repo):
                 movemark = repo['.'].node()
+    return node, movemark, None
+
+def destupdate(repo, clean=False, check=False):
+    """destination for bare update operation
+
+    return (rev, movemark, activemark)
+
+    - rev: the revision to update to,
+    - movemark: node to move the active bookmark from
+                (cf bookmark.calculate update),
+    - activemark: a bookmark to activate at the end of the update.
+    """
+    node = None
+    wc = repo[None]
+    movemark = activemark = None
+
+    node, movemark, activemark = _destupdateobs(repo, clean, check)
 
     if node is None:
         # we also move the active bookmark, if any