filemerge: support specifying a python function to custom merge-tools Eliminates the need to specify a python executable, which may not exist on system. Additionally launching script inprocess aids portability on systems that can't execute python via the shell. Example usage "merge-tools.myTool.executable=python:c:\myTool.py:mergefn" where myTool.py contains a function: "def mergefn(ui, repo, args, **kwargs):" where args is list of args passed to merge tool. (by default, expanded: $local $base $other) Invoking the specified python function was done by exposing and invoking (hook._pythonhook -> hook.pythonhook)