clone/update: feed git-svn a file of empty lines

clone/update: feed git-svn a file of empty lines

The `git svn` command has a nasty habit of prompting the
user whenever anything unusual happens.

The `svn` command line has a `--non-interactive` option to
prevent exactly this sort of hang, but `git svn` does not.

Fortunately, simply pressing "<return>" will prevent the
hang (usually the default action causes a failure result).

Feed a file of empty lines to every `git svn init` and
`git svn fetch` command to satisfy these unwanted prompts.

This should prevent any svn mirror fetches from hanging
indefinitely until they are killed by the jobd timeout.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>