Add failing test_push_after_merge

[jedbrown]

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.