destupdate: extract logic based on branch 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

@@ -85,6 +85,21 b' def _destupdatebook(repo, clean, check):'
         activemark = node
     return node, movemark, activemark
 
+def _destupdatebranch(repo, clean, check):
+    """decide on an update destination from current branch"""
+    wc = repo[None]
+    movemark = node = None
+    try:
+        node = repo.branchtip(wc.branch())
+        if bookmarks.isactivewdirparent(repo):
+            movemark = repo['.'].node()
+    except error.RepoLookupError:
+        if wc.branch() == 'default': # no default branch!
+            node = repo.lookup('tip') # update to tip
+        else:
+            raise error.Abort(_("branch %s not found") % wc.branch())
+    return node, movemark, None
+
 def destupdate(repo, clean=False, check=False):
     """destination for bare update operation
 
@@ -96,22 +111,13 b' def destupdate(repo, clean=False, check='
     - 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:
         node, movemark, activemark = _destupdatebook(repo, clean, check)
-
     if node is None:
-        if node is None:
-            try:
-                node = repo.branchtip(wc.branch())
-            except error.RepoLookupError:
-                if wc.branch() == 'default': # no default branch!
-                    node = repo.lookup('tip') # update to tip
-                else:
-                    raise error.Abort(_("branch %s not found") % wc.branch())
+        node, movemark, activemark = _destupdatebranch(repo, clean, check)
     rev = repo[node].rev()
 
     _destupdatevalidate(repo, rev, clean, check)