scripts/bundle: use git instead of tar.gz

scripts/bundle: use git instead of tar.gz

To better fit with our current CI processes, use git to clone the
suricata-update and libhtp dependencies.  The requirements.txt file has
been modified to take a repo URL and a `-b` command line option for tag
or branch.

For the 6.0.x branch we will use the libhtp 0.5.x branch and the
suricata-update 1.2.4 tag.

Also allows for repo and branch names to be overrided with environment
variables:
- SU_REPO
- SU_BRANCH
- LIBHTP_REPO
- LIBHTP_BRANCH