tests/test-issue619.t

py3: conditionalize the raise statement

raise E,V,T is not acceptable in Python 3, thats is conditionalized.
Moreover this will result in syntax error so we have to use exec() to
execute this. Related PEP- https://www.python.org/dev/peps/pep-3109/#id14

My implementation is motivated from the six implementation except they are
defining a new function exec_() to prevent adding an extra frame AFAIK :)

https://bitbucket.org/gutworth/six/src/ca4580a5a648/six.py#six.py-680

https://bz.mercurial-scm.org/619

  $ 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]

Even with strange revset (issue4465)

  $ hg merge ::.
  abort: merging with a working directory ancestor has no effect
  [255]