tests: avoid unexpected result at invocation of *.py file on Windows Invocation of "diff tool.py" in test-extdiff.t tests whether shellquote() is applied on specified command as expected. But direct invocation of "*.py" file might cause unexpected result on Windows according to suffix binding. For example, starting IDE, showing dialog to choose program to be used, and so on. In such case, running test-extdiff.t is easily timed out. This patch uses intermediate *.bat file on Windows, to avoid such unexpected result. Naming that intermediate file as "diff tool.bat" is enough to test applying shellquote().