R1:f5f784f4c1d0

Version clustered, observed repositories in a reasonable way (by largest discovered HEAD)

Summary:
Ref T4292. For hosted, clustered repositories we have a good way to increment the internal version of the repository: every time a user pushes something, we increment the version by 1.

We don't have a great way to do this for observed/remote repositories because when we `git fetch` we might get nothing, or we might get some changes, and we can't easily tell //what// changes we got.

For example, if we see…