fix: prevent fatal clone/fetch errors when PR git data unavailable

Only advertise `refs/heads/pr/*` branches once their tip OIDs are
confirmed present locally; prevents `fatal: bad object` / `remote did
not send all necessary objects` errors during clone/fetch when a PR tip
lives on a different git server than the one that won the bulk prefetch
race.

After the bulk prefetch, collect remaining missing PR tip OIDs and do
one batch fetch per repo git server using only the OIDs that server has
advertised; break early once all are satisfied and skip servers that
carry none. Avoids batch-poisoning (a server rejects the whole request
if any single OID is absent) and redundant connections.

Restrict mop-up fetches and run_fetch to the repo's declared git
servers; do not fetch from clone-tag URLs in PR events - they are
submitter-supplied and could let a malicious or slow server stall every
clone/fetch operation.

Also apply rustfmt and fix clippy warnings.

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