http://mercurial.selenic.com/bts/issue619

  $ hg init

  $ echo a > a

  $ hg ci -Ama

  adding a

  $ echo b > b

  $ hg branch b

  marked working directory as branch b

  (branches are permanent and global, did you want a bookmark?)

  $ hg ci -Amb

  adding b

  $ hg co -C 0

  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

Fast-forward:

  $ hg merge b

  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

  (branch merge, don't forget to commit)

  $ hg ci -Ammerge

Bogus fast-forward should fail:

  $ hg merge b

  abort: merging with a working directory ancestor has no effect

  [255]