destupdate: include the 'check' logic

destupdate: include the 'check' logic

After moving logic from 'merge.update' into 'destutil.destupdate', we are now
moving logic from 'command.update' in 'destutil.destupdate'. This will make the
function actually useful in predicting (and altering) the update behavior.

--- a/mercurial/commands.py	Mon Oct 05 03:50:47 2015 -0700
+++ b/mercurial/commands.py	Tue Sep 29 00:18:49 2015 -0700
@@ -6595,10 +6595,8 @@
 
         if check:
             cmdutil.bailifchanged(repo, merge=False)
-            if rev is None:
-                rev = repo[repo[None].branch()].rev()
-        elif rev is None:
-            rev = destutil.destupdate(repo, clean=clean)
+        if rev is None:
+            rev = destutil.destupdate(repo, clean=clean, check=check)
 
         repo.ui.setconfig('ui', 'forcemerge', tool, 'update')
 

--- a/mercurial/destutil.py	Mon Oct 05 03:50:47 2015 -0700
+++ b/mercurial/destutil.py	Tue Sep 29 00:18:49 2015 -0700
@@ -11,7 +11,7 @@
     obsolete,
 )
 
-def destupdate(repo, clean=False):
+def destupdate(repo, clean=False, check=False):
     """destination for bare update operation
     """
     # Here is where we should consider bookmarks, divergent bookmarks, and tip
@@ -71,7 +71,7 @@
                     hint = _("commit and merge, or update --clean to"
                              " discard changes")
                     raise error.Abort(msg, hint=hint)
-                else:  # destination is not a descendant.
+                elif not check:  # destination is not a descendant.
                     msg = _("not a linear update")
                     hint = _("merge or update --check to force update")
                     raise error.Abort(msg, hint=hint)