pkg-download: Make a shallow clone for git downloads

When downloading from git, we clone the whole repository and then only
use the latest commit.  That's a lot of redundant stuff.  So instead,
make a shallow clone.  Unfortunately that's only possible when
downloading a branch or tag, so fall back to the old method if git gives
an error.

This speeds up the cloning of a linux git from more than 2 hours to
20 minutes on a 200KB/s link).

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>