update: localize logic around which ancestor to use

The merge code works by applying the changes between an ancestor and
the working copy onto the destination. To achieve an update, it sets
the ancestor to be the parent of the working copy. To achieve a clean
update (update -C), it sets the ancestor to be the working copy itself
(so there are no changes to carry over). The logic for this was spread
out a bit. Let's move it all to one place so it's easier to follow the
reason for it. Also add some documentation.

mercurial/merge.py

@@ -1556,7 +1556,7 b' def update(repo, node, branchmerge, forc'
                     foreground = obsolete.foreground(repo, [p1.node()])
                     # note: the <node> variable contains a random identifier
                     if repo[node].node() in foreground:
-                        pas = [p1]  # allow updating to successors
+                        pass # allow updating to successors
                     elif dirty:
                         msg = _("uncommitted changes")
                         if onode is None:
@@ -1572,15 +1572,17 b' def update(repo, node, branchmerge, forc'
                         raise error.Abort(msg, hint=hint)
                 else:
                     # Allow jumping branches if clean and specific rev given
-                    pas = [p1]
+                    pass
+
+        if overwrite:
+            pas = [wc]
+        elif not branchmerge:
+            pas = [p1]
 
         # deprecated config: merge.followcopies
         followcopies = repo.ui.configbool('merge', 'followcopies', True)
         if overwrite:
-            pas = [wc]
             followcopies = False
-        elif pas == [p2]: # backwards
-            pas = [p1]
         elif not pas[0]:
             followcopies = False
         if not branchmerge and not wc.dirty(missing=True):