bug#11761: Slight bug in split :-)

[Paul Eggert]

>From a filesystem point of view, would it be
more efficient to invoke ftruncate at the end of
writing, rather than at the beginning?  That way,
if the file already exists and is of the right size,
it won't need to be reallocated.  We're not trying
to write any holes, so this optimization should be
valid.

Please don't let this comment slow you down, as your
patch is fine as-is.  I'm mainly asking because I was
wondering about the issue in general.