test-merge-tools: windows fixes
- /bin/cat does not exist, expect 'cat'
- non-interactive mode is not correctly detected in pysh, force it
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'