suport/download: fix git wrapper with submodules on older git versions

suport/download: fix git wrapper with submodules on older git versions

Older versions of git store the absolute path of the submodules'
repository as stored in the super-project, e.g.:

    $ cat some-submodule/.git
    gitdir: /path/to/super-project/.git/modules/some-submodule

Obviously, this is not very reproducible.

More recent versions of git, however, store relative paths, which
de-facto makes it reproducible.

Fix older versions by replacing the absolute paths with relative ones.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>