windows: expand environment vars in find_exe

This allows us to use ${ProgramFiles} in our [merge-tools] sections so
we can provide locale safe program paths.

mercurial/windows.py

@@ -203,7 +203,7 b' def find_exe(command):'
         executable = findexisting(os.path.join(path, command))
         if executable is not None:
             return executable
-    return None
+    return findexisting(os.path.expanduser(os.path.expandvars(command)))
 
 def set_signal_handler():
     try: