unix: more fixes for the Git_fs.IO on Windows
unix: more fixes for the Git_fs.IO on Windows

It seems that unlink can also raise EACCES on Windows (see
https://msdn.microsoft.com/en-us/library/5814770t.aspx)

Also, the current exponential backoff for writing files was lacking
intermediate steps, which would make some write very slow.

mirage/ocaml-git,mirage/ocaml-git,mirage/ocaml-git