Repository mirroring

Fixes: #8 (closed)

• Pull mirroring
  • Pull mirroring CI
  • Check dry-run

Out of scope for now:

• Pull mirroring:
  • Ability to disable pipeline on push (coded, but depends on libgit2#5335)
  • Ability to force-push
• Push mirroring
• Other VCS (bzr, svn, cvs, ...)