QA: helper to watch for repository replication instead of using sleep

The following discussion from !3893 (merged) should be addressed:

• @godfat started a discussion: (+1 comment)

  Is there any way we could detect if this completes?

• @brodock we need to code a helper for that, it would be roughly like:

• define a git_url

• define a pool_wait (ms)

• define a timeout (ms or seconds)

• define a commit_hash

1. try to git ls-remote #{git_url} | grep commit_hash

2. wait for pool_wait if failed and retry

3. abort if timeout