contrib: Fix onnx build, which depends on the repo metadata.

Commit 8e2b334d normalized how the onnx sources are fetched; compared
to earlier code, it strips the .git metadata.  While usually a nice
thing to do (as the .git metadata is not fixed and thus a checksum for
the resulting tarball archive cannot be computed), in this case it
breaks the build, which relies on it.

The solution is to add a 4th argument to the 'download_git' procedure,
that allows preserving the .git repository metadata.

* contrib/src/main.mak (download_git): Add a new argument and document it.
* contrib/src/onnx/rules.mak
($(TARBALLS)/onnxruntime-$(ONNX_VERSION).tar.xz): Use it.

Reported-by: Aline <aline.gondimsantos@savoirfairelinux.com>
Change-Id: Ia5a5dad2a8dd2c292b8b8d8b8e5f63361a9484ef