bookmarks: restrict moving a bookmark to its descendants (issue1502)

A bookmark can only move to a descendant on commit, pull or
unbundle. Bookmarks cannot jump between heads anymore. This fixese
issue 1502.

We explicitly use new.node(), to emphasise that we are updating the
current bookmark to the new node.

mercurial/bookmarks.py

@@ -127,8 +127,11 @@ def update(repo, parents, node):
     update = False
     mark = repo._bookmarkcurrent
     if mark and marks[mark] in parents:
-        marks[mark] = node
-        update = True
+        old = repo[marks[mark]]
+        new = repo[node]
+        if new in old.descendants():
+            marks[mark] = new.node()
+            update = True
     if update:
         write(repo)
 

tests/test-issue1502.t

@@ -0,0 +1,42 @@
+http://mercurial.selenic.com/bts/issue1502
+
+Initialize repository
+
+  $ hg init foo
+  $ touch foo/a && hg -R foo commit -A -m "added a"
+  adding a
+
+  $ hg clone foo foo1
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ echo "bar" > foo1/a && hg -R foo1 commit -m "edit a in foo1"
+  $ echo "hi" > foo/a && hg -R foo commit -m "edited a foo"
+  $ hg -R foo1 pull -u
+  pulling from $TESTTMP/foo
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  not updating, since new heads added
+  (run 'hg heads' to see heads, 'hg merge' to merge)
+
+  $ hg -R foo1 book branchy
+  $ hg -R foo1 book
+   * branchy                   1:e3e522925eff
+
+Pull. Bookmark should not jump to new head.
+
+  $ echo "there" >> foo/a && hg -R foo commit -m "edited a again"
+  $ hg -R foo1 pull
+  pulling from $TESTTMP/foo
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  (run 'hg update' to get a working copy)
+
+  $ hg -R foo1 book
+   * branchy                   1:e3e522925eff