[PATCH RFC] update: default update should move as far forward as possible (issue3883)

[Durham Goode]

# HG changeset patch
# User Durham Goode <durham at fb.com>
# Date 1365614989 25200
#      Wed Apr 10 10:29:49 2013 -0700
# Node ID 4f8c54c2540d1a28773f0dd48e6e2b92163259e4
# Parent  dfd94b32f8731515a0e6e90642609343dc1d6d4d
update: default update should move as far forward as possible (issue3883)

DO NOT QUEUE THIS CHANGE! It's an rfc.

Previously the default behavior for 'hg up' was to move to the tip of the
current named branch. This meant if you were on a different fork of the
DAG from the tip you got an "abort: crosses branches" error.

This changes 'hg up' to update to the farthest descendant of your current
working copy parent within your current named branch. Ex:

o  3
|
| o  2
| |
| @  1
|/
|
o  0

'hg update' will now put you on commit 2.  If you were already on commit 2,
it would be a no-op.  If you were on commit 0, it would put you on 3.

This only affects hg updates that would have previously failed. hg updates
that would have succeeded before will continue to succeed and land on tip.

I considered making this only happen if they have an active bookmark, but
it seemed weird to have different behavior based on if your bookmark was
active or not, and it seems like the new behavior is desirable even in non
bookmark scenarios.

If people agree with this change, I'll update the tests and help documentation
and resubmit.

diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -652,14 +652,17 @@
     try:
         wc = repo[None]
         if node is None:
-            # tip of current branch
-            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 util.Abort(_("branch %s not found") % wc.branch())
+            # move as far forward as possible
+            wcbranch = wc.branch()
+            nodectx = wc.parents()[0]
+            descendants = nodectx.descendants()
+            for descendant in descendants:
+                if (descendant.branch() == wcbranch and
+                    descendant.rev() > nodectx.rev()):
+                    nodectx = descendant
+
+            node = nodectx.node()
+
         overwrite = force and not branchmerge
         pl = wc.parents()
         p1, p2 = pl[0], repo[node]