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().

tests/test-extdiff.t

@@ -263,8 +263,16 b' Prepare custom diff/edit tool:'
 will change to /tmp/extdiff.TMP and populate directories a.TMP and a
 and start tool
 
+#if windows
+  $ cat > 'diff tool.bat' << EOF
+  > @$PYTHON "`pwd`/diff tool.py"
+  > EOF
+  $ hg extdiff -p "`pwd`/diff tool.bat"
+  [1]
+#else
   $ hg extdiff -p "`pwd`/diff tool.py"
   [1]
+#endif
 
 Diff in working directory, after: