[git-ref] change edits after prepare()

[Byron]

Find a helpful API do help in these cases:

radicle-dev/radicle-link#761 (comment)

[Byron]

CC @kim I will implement as part of the still ongoing 🤦‍♂️ git fetch work block as I think it's valuable to lock the refs-to-be-fetched to prevent racy fetches, and change the tips to the then known location once the fetch is complete. This would naturally prevent fetching the same heads in multiple processes as well.

[Byron]

Let's rather retry even if this means that potentially expensive computation has to be redone.
For now we assume such optimizations aren't worth it, but it's OK to revisit this once actual data from a busy server is available.