Fix windows test failure in test_show_revision_for_merge

The test_show_revision_for_merge fails on Windows with the JGit implementation.  It succeeds on Windows with the CLI implementation and it succeeds on Linux with both implementations.

This change removes a trailing ^M character from the file name which is collected from diff output during test execution.  The JGit diff output includes the ^M character in the file name, while the other three configurations do not.

Since the diff output comparison is internal to the test, I assume that it is better to fix the test infrastructure than to attempt a change to the JGit diff output on Windows.