win32: use explicit path to "python.exe" only if it exists Before this patch, "hg.bat" for Windows environment always uses "%~dp0..\python" as explicit path to "python.exe". This path may not be valid in some cases. For example, on the environment using "virtualenv" python package, both "python.exe" and "hg.bat" are placed in the same directory. In this case, "python.exe" should be found on PATH, because virtualenv activation script puts "python.exe" on the PATH. This patch uses explicit path to "python.exe" only if it exists, and expects that "python.exe" can be found on PATH otherwise.