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.