mailinfo: handle_commit_msg() shouldn't be called after finding patchbreak

There is a strange "if (!mi->cmitmsg) return 0" at the very beginning of handle_commit_msg(), but the condition should never trigger, because: * The only place cmitmsg is set to NULL is after this function sees a patch break, closes the FILE * to write the commit log message and returns 1. This function returns non-zero only from that codepath. * The caller of this function, upon seeing a non-zero return, increments filter_stage, starts treating the input as patch text and will never call handle_commit_msg() again. Replace it with an assert(!mi->filter_stage) to ensure the above observation will stay to be true. Signed-off-by: Junio C Hamano <gitster@pobox.com>