provide archives of foreign git repositories via http(s)

Installation:

- make "work/wish-list" writable for webserver user
- point a php-capable webserver to httpdocs
- run clone-repos by cronjob or systemd timer
- run update-repos by cronjob or systemd timer

Use:

access on the webserver:

- to schedule for cloning
  ?t=git/hg&r=base64_encode(https://url.to/upstream/repository.git)

- to retrieve tar ball:
  ?t=git/hg&r=base64_encode(https://url.to/upstream/repository.git)&tag=base64_encode(tag)
  ?t=git/hg&r=base64_encode(https://url.to/upstream/repository.git)&commit=hash
  ?t=git&r=base64_encode(https://url.to/upstream/repository.git)&commit=hash&valid_keys=fp1,fp2,fp3