Improve repo-updater tracing

Created by: beyang

This adds additional tracing to debug 2 issues experienced by a customer:

• enqueue-repo-update requests would fail with HTTP 500 statuses. Adding additional logging resolved the issue unexpectedly, suggesting a race condition.
• repo-updater was observed to hang on startup (it never reached this line: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@7bdaddd1bfed9c2b2a6395084cb8e543d0b11fd5/-/blob/cmd/repo-updater/main.go?suggestion#L195)

Post-merge TODO

• Cherry-pick onto 3.4 and cut a patch release.