extdiff: refactor closure of saved diff command as a top-level class This allows us to collect __doc__ for translation.

i18n: use inspect.getsourcelines() to obtain lineno from func or class Before, func must be a function object. I want to make it parse docstring of classes, too.

chg: just take it as EOF if recv() returns 0 hgc->sockfd is a blocking stream socket. recv() should never return 0 other than EOF. See 4fc4b8cc9957 for the original problem.