pkg-infra: move the git download helper to a script

The git download helper is getting a bit more complex. Fixing it in the
Makefile when it breaks (like the recent breakage with a non-existing
sha1-cset) proves to be challenging, to say the least.

Move it into a shell script in support/download/git, which will make
it much easier to read, maintain, fix and enhance in the future.

[Peter: redirect pushd/popd output to /dev/null]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>