testing/tests/download: test git branch

Add a branch to the static repo and check on the git refs test case the
download of a git package:
 - with a sha1 reachable by a branch name, but not pointed by it, as
   version. This is the most common use case for git refs in the tree;
 - with a partial sha1 of a commit reachable by a branch as version;
 - with a sha1 of the commit head of a branch as version;
 - with a partial sha1 of the commit head of a branch as version;

Enforce the download always occurs by removing the BR2_DL_DIR used for
the tarballs generated by the git download infra.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Cc: Arnout Vandecappelle <arnout@mind.be>
Tested-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>