Don't be fooled by strings.Split(..., "\n") result always having empty "" last element

By definition of strings.Split(..., sep) it "slices s into all substrings separated by sep and returns a slice of the substrings between those separators". That means that

string.Split("hello\nworld\n", "\n") -> ["hello", "world", ""])     # NOTE the last ""

when parsing file by lines, it is handy though to do not get last empty "" after last "\n". #6 (closed) shows how we missed to do that filtering-out for case of empty backup.refs file and errored-out because of that.

To fix let's introduce a helper - splitlines(), which does the job of filtering-out last empty entry after last separator. By using this helper everywhere we can hopefully avoid problems while pulling only empty repositories (#6 (closed) case), and also similar ones.

Fixes #6 (closed) /reported-by @iv