download: git: introduce cache feature

download: git: introduce cache feature

Now we keep the git clone that we download and generates our tarball
from there.
The main goal here is that if you change the version of a package (say
Linux), instead of cloning all over again, you will simply 'git fetch'
from the repo the missing objects, then generates the tarball again.

This should speed the 'source' part of the build significantly.

The drawback is that the DL_DIR will grow much larger; but time is more
important than disk space nowadays.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>