Work around curl bug when asked to download an empty url to a file.

In this situation, curl -o exits successfully without creating the output
file.

There was already a workaround for curl file:/// but I did not realize this
also affected regular url downloads.

To fix it, pre-create the destination file before starting curl.
Since we cannot always know the size of an url before trying to download
it, let's always do this.

Note that since curl is told -C -, we have to consider if this
makes curl try to do a ranged download, which might fail on some servers
where a regular download would have succeeded. My testing indicates
this isn't a problem; since the file is empty, curl seems to not try to
do a ranged download.

Original report: https://github.com/datalad/datalad/issues/79
Curl bug report: https://github.com/bagder/curl/issues/183

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