hgforest.sh fails to detect Python correctly on MacOS

With the mac ports version of mercurial the path to python in the mercurial launcher has is

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python

Note the capital "P" which is what was running afoul of the hgforest.sh detection. A different technique for checking the python executable is needed