Changeset diff render can fail when gitserver hasn't synced yet

Created by: eseliger

When I update a changesets branch, the webhook updates the metadata. Hence, the head oid changes. When a changeset fetch occurs with an expanded changeset the diff fails when there’s a new commit, because gitserver doesn’t have the commit yet

Git command [git merge-base -- 0440892751ca39eb2cdbf88ca2e4e42ea9252cb4 d3647d131ff88f2fe3393d1cb20b6de208f39c5c] failed (output: "fatal: Not a valid commit name d3647d131ff88f2fe3393d1cb20b6de208f39c5c"): exit status 128

I think we should ensure the revision exists before we return the diff for it.