merge: consider successor changesets for a bare update

Previously, a bare update would ignore any successor changesets thus
potentially leaving you on an obsolete head. This happens commonly when there
is an old bookmark that hasn't been moved forward which is the motivating
reason for this patch series.

Now, we will check for successor changesets if two conditions hold: 1) we are
doing a bare update 2) *and* we are currently on an obsolete head.

If we are in this situation, then we calculate the branchtip of the successor
set and update to that changeset.

Tests coverage has been added.

mercurial/merge.py

@@ -696,6 +696,34 b' def update(repo, node, branchmerge, forc'
                     node = repo.lookup("tip") # update to tip
                 else:
                     raise util.Abort(_("branch %s not found") % wc.branch())
+
+            if p1.obsolete() and not p1.children():
+                # allow updating to successors
+                successors = obsolete.successorssets(repo, p1.node())
+
+                # behavior of certain cases is as follows,
+                #
+                # divergent changesets: update to highest rev, similar to what
+                #     is currently done when there are more than one head
+                #     (i.e. 'tip')
+                #
+                # replaced changesets: same as divergent except we know there
+                # is no conflict
+                #
+                # pruned changeset: no update is done; though, we could
+                #     consider updating to the first non-obsolete parent,
+                #     similar to what is current done for 'hg prune'
+
+                if successors:
+                    # flatten the list here handles both divergent (len > 1)
+                    # and the usual case (len = 1)
+                    successors = [n for sub in successors for n in sub]
+
+                    # get the max revision for the given successors set,
+                    # i.e. the 'tip' of a set
+                    node = repo.revs("max(%ln)", successors)[0]
+                    pa = p1
+
         overwrite = force and not branchmerge
 
         p2 = repo[node]

tests/test-update-branches.t

@@ -229,6 +229,14 b' the bookmark (issue4015)'
   $ hg bookmarks
    * bm                        5:ff252e8273df
 
+Test that 4 is detected as the no-argument destination from 3
+  $ hg up --quiet 0          # we should be able to update to 3 directly
+  $ hg up --quiet --hidden 3 # but not implemented yet.
+  $ hg up
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  d047485b3896+ (b1)
+
 Test that 5 is detected as a valid destination from 1
   $ hg up --quiet 0          # we should be able to update to 3 directly
   $ hg up --quiet --hidden 3 # but not implemented yet.
@@ -242,3 +250,14 b' Test that 5 is not detected as a valid d'
   abort: uncommitted changes
   (commit or update --clean to discard changes)
   [255]
+
+Test that we don't crash when updating from a pruned changeset (i.e. has no
+successors). Behavior should probably be that we update to the first
+non-obsolete parent but that will be decided later.
+  $ hg id --debug -r 2
+  bd10386d478cd5a9faf2e604114c8e6da62d3889
+  $ hg up --quiet 0
+  $ hg up --quiet 2
+  $ hg debugobsolete bd10386d478cd5a9faf2e604114c8e6da62d3889
+  $ hg up
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved