In gsubmodd_test.py (whose structure was modeled after gsubtreed_test.py) we capture stdout by redirecting it to a file.  However, the way it is now, if we add a Python `print` statement somewhere in the execution path (for temporary debugging), all hell breaks loose: Git operations start breaking, which should have nothing to do with the added debugging output.

Opening the stdout capture file for 'w+' instead of the default read-only eliminates the problem.