- pass an argument for the file into which we want to save the result, to
avoid getting it polluted with error messages (defaults to /dev/stdout for
debug).
- create the makefile fragment as a temp file and only copy it when complete.
- copy it in one chunk, so that one can read a partial mirror-maker file
and have it be usable.
This does allow for people to start a make mirror-maker in one shell, and
start fetching stuff right away, before mirror-maker is finished.
This also produces usable mirror-maker Makefiles even if the ports tree
contains bogus entries.
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