test-merge-tools: windows fixes
- /bin/cat does not exist, expect 'cat'
- non-interactive mode is not correctly detected in pysh, force it
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!