destupdate: extract logic based on bookmarks 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

@@ -76,6 +76,15 b' def _destupdateobs(repo, clean, check):'
                 movemark = repo['.'].node()
     return node, movemark, None
 
+def _destupdatebook(repo, clean, check):
+    """decide on an update destination from active bookmark"""
+    # we also move the active bookmark, if any
+    activemark = None
+    node, movemark = bookmarks.calculateupdate(repo.ui, repo, None)
+    if node is not None:
+        activemark = node
+    return node, movemark, activemark
+
 def destupdate(repo, clean=False, check=False):
     """destination for bare update operation
 
@@ -91,13 +100,10 b' def destupdate(repo, clean=False, check='
     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:
-        # we also move the active bookmark, if any
-        node, movemark = bookmarks.calculateupdate(repo.ui, repo, None)
-        if node is not None:
-            activemark = node
-
         if node is None:
             try:
                 node = repo.branchtip(wc.branch())