destupdate: have a generic and extensible way to run each step

We want extension to be able to easily override or add new way to select the
default update destination. We use the same list + dict approach as in other
parts of the code.

mercurial/destutil.py

@@ -100,6 +100,15 b' def _destupdatebranch(repo, clean, check'
             raise error.Abort(_("branch %s not found") % wc.branch())
     return node, movemark, None
 
+# order in which each step should be evalutated
+# steps are run until one finds a destination
+destupdatesteps = ['evolution', 'bookmark', 'branch']
+# mapping to ease extension overriding steps.
+destupdatestepmap = {'evolution': _destupdateobs,
+                     'bookmark': _destupdatebook,
+                     'branch': _destupdatebranch,
+                     }
+
 def destupdate(repo, clean=False, check=False):
     """destination for bare update operation
 
@@ -110,14 +119,12 b' def destupdate(repo, clean=False, check='
                 (cf bookmark.calculate update),
     - activemark: a bookmark to activate at the end of the update.
     """
-    node = None
-    movemark = activemark = None
+    node = 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:
-        node, movemark, activemark = _destupdatebranch(repo, clean, check)
+    for step in destupdatesteps:
+        node, movemark, activemark = destupdatestepmap[step](repo, clean, check)
+        if node is not None:
+            break
     rev = repo[node].rev()
 
     _destupdatevalidate(repo, rev, clean, check)