support/download: fix the git helper

When switching the git helper over to a shell script, a special case was
not carried over: in case the remote has the required reference, we
attempt a shallow clone, using --depth 1. However, this is not supported
when the remote is accessed with the http protocol.

Therefore, the download fails.

What happened before the conversion to a shell script was that the helper
in the Makefile would fallback to doing a full-clone.

This is the case and behaviour that were lost in the conversion.

To avoid making the script too complex, we only attempt a full clone if
needed. And we decide that a full clone is needed by default; we decide
it is unnecessary if the remote has the needed reference *and* the
shallow clone was successful.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>