Add failing test_push_after_merge

related test_push_after_rebase() passes

This currently leaves `.git/hg/origin/clone` with multiple heads:

    test_push_after_merge0/git_dir/hg_base/.git/hg/origin/clone$ hg log --graph
    o  changeset:   2:24d9eafbdfe8
    |  tag:         tip
    |  parent:      0:b26b618dcf12
    |  user:        Jed Brown
    |  date:        Wed Jan 23 21:36:41 2013 -0600
    |  summary:     c
    |
    | o  changeset:   1:5d4d96dc7a10
    |/   user:        Jed Brown <jed@59A2.org>
    |    date:        Wed Jan 23 21:36:41 2013 -0600
    |    summary:     b
    |
    o  changeset:   0:b26b618dcf12
       user:        Jed Brown <jed@59A2.org>
       date:        Wed Jan 23 21:36:40 2013 -0600
       summary:     a

The faulty author field is fixed by issue #17, but this issue persists.