bookmarks: consider successor changesets when moving bookmark (issue4015)

Previously, this required -f because we didn't consider obsolete changesets
(and their children ... or successors of those children, etc.). We now use
obsolete.foreground to calculate acceptable changesets when advancing the
bookmark.

Test coverage has been added.

mercurial/commands.py

@@ -836,10 +836,12 b' def bookmark(ui, repo, *names, **opts):'
                     bookmarks.deletedivergent(repo, [target], mark)
                     return
 
+                # consider successor changesets as well
+                foreground = obsolete.foreground(repo, [marks[mark]])
                 deletefrom = [b for b in divs
                               if repo[b].rev() in anc or b == target]
                 bookmarks.deletedivergent(repo, deletefrom, mark)
-                if bmctx.rev() in anc:
+                if bmctx.rev() in anc or target in foreground:
                     ui.status(_("moving bookmark '%s' forward from %s\n") %
                               (mark, short(bmctx.node())))
                     return

tests/test-update-branches.t

@@ -206,6 +206,7 b' Test no-argument update to a successor o'
   |/
   o  0:60829823a42a 0
   
+  $ hg book bm -r 3
   $ hg status
   M foo
 
@@ -218,10 +219,16 b' We add simple obsolescence marker betwee'
   $ hg debugobsolete 6efa171f091b00a3c35edc15d48c52a498929953 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   $ hg debugobsolete aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa d047485b3896813b2a624e86201983520f003206
 
-Test that 5 is detected as a valid destination from 3
+Test that 5 is detected as a valid destination from 3 and also accepts moving
+the bookmark (issue4015)
+
   $ hg up --quiet --hidden 3
   $ hg up 5
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg book bm
+  moving bookmark 'bm' forward from 6efa171f091b
+  $ hg bookmarks
+   * bm                        5:ff252e8273df
 
 Test that 5 is detected as a valid destination from 1
   $ hg up --quiet 0          # we should be able to update to 3 directly