tests/test-username-newline.out

prepush: add more precise error messages

Part of the patch is from [e-mail]

- indicate the branch name where there are multiple heads
- give better advice when hitting a possible race, where new heads are added
  between discovery and the call to branchmap(). In that case, asking the user
  to merge isn't helpful, since only remote has the changes.

adding a
abort: username 'foo\nbar1' contains a newline

abort: username 'foo\nbar2' contains a newline

transaction abort!
rollback completed
abort: username 'foo\nbar3' contains a newline!