Re: [PATCH v2 5/7] merge-ll: use read_mmfile() to read external merge results

> After running an external merge driver, ll_ext_merge() reads the result
> back from a temporary file. We can do the same thing with much less code
> by using read_mmfile().
> 
> There are also two behavior improvements.
> 
> One, read_mmfile() correctly uses xsize_t() to detect the case when we'd
> truncate the result.
> 
> And two, read_mmfile() will report errors to stderr if it can't read the
> file (whereas the existing code silently returned NULL). I think most
> callers would have said _something_ in this case like "failed to execute
> merge" (from merge-ort), but more specifics are probably helpful (e.g.,
> to distinguish a random system error from a badly configured merge
> driver).