pkg-download.mk: Fix shallow clone

Some git versions (e.g. 1.7.0) do not treat trying to shallow clone
a non existing branch or tag as a fatal error but report a warning
and clone HEAD instead. Thus the fallback mechanism does not work
in this case.

This patch introduces a check for the presence of the requested
version as a branch or tag before trying the shallow clone. It
also removes the need to do two clones when a sha1 is given as
a packege version.

[Peter: use cut -f2-]
Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>