When launching an external special remote, use the shebang handling code which currently exists elsewhere in git-annex

[joeyh] """Oh, git-annex already deals with this particular windows nonsense elsewhere. When it needs to run a git hook, it parses it for a shebang. Git for windows does the same.

So, if you can please open a todo item in git-annex, I can refactor that existing code to be used in more places."""

done --Joey