Fix OSV sync shallow clone failing on quiet weekends (#43450)

## Summary

The nightly OSV artifact generation in `fleetdm/vulnerabilities` failed
over the weekend with:

```
fatal: error processing shallow info: 4
```

at `cmd/osv-processor/sync-and-detect-changes.sh` during:

```bash
git fetch --shallow-since="3 days ago" origin main
```

Root cause: `git fetch --shallow-since` errors out when the upstream
(`canonical/ubuntu-security-notices`) has zero commits newer than the
cutoff. Canonical didn't push anything over the weekend, so the 3-day
window returned empty and upload-pack produced an unusable shallow
response.

Fix:
- Fall back to `git fetch --depth=3` if `--shallow-since` still returns
empty, so the initial clone always succeeds.

Subsequent runs reuse the existing clone and take the other branch of
the script (plain `git fetch origin main`), which doesn't have this
failure mode.

Failing run:
https://github.com/fleetdm/vulnerabilities/actions/runs/24330589309/job/71035337352

## Test plan

- [x] Re-run the Ubuntu OSV artifact generation workflow; initial clone
succeeds regardless of upstream push frequency.
- [x] Manually exercise the cold-cache path locally: `rm -rf
ubuntu-security-notices &&
./cmd/osv-processor/sync-and-detect-changes.sh` — completes without
error.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Improved initial repository sync: if the primary shallow fetch returns
no commits, the process now falls back to a limited-depth fetch, warns
the user, and shows recent commit history before continuing. Downstream
change detection and existing behavior for already-cloned repos remain
unchanged.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Konstantin Sykulev <konst@sykulev.com>

cmd/osv-processor/sync-and-detect-changes.sh

@@ -16,6 +16,10 @@ set -euo pipefail
 REPO_URL="https://github.com/canonical/ubuntu-security-notices.git"
 REPO_DIR="ubuntu-security-notices"
 DAYS_TO_KEEP=3 # how much git history to get for initial clone
+# Fallback depth used when --shallow-since returns no commits (upstream was quiet).
+# The sync only needs the tip commit to populate the working tree.
+# We grab a few extra commits so the warning log shows recent upstream activity for debugging.
+FALLBACK_DEPTH=3
 
 echo "=== OSV Repository Sync ==="
 echo ""
@@ -57,7 +61,17 @@ else
     git config core.sparseCheckout true
     echo "osv/" > .git/info/sparse-checkout
 
-    git fetch --shallow-since="${DAYS_TO_KEEP} days ago" origin main
+    if ! git fetch --shallow-since="${DAYS_TO_KEEP} days ago" origin main; then
+        echo ""
+        echo "WARNING: --shallow-since=${DAYS_TO_KEEP}d returned no commits."
+        echo "Upstream has been quiet for >${DAYS_TO_KEEP} days. Falling back to --depth=${FALLBACK_DEPTH}."
+        echo ""
+        git fetch --depth="${FALLBACK_DEPTH}" origin main
+        echo "Recent history:"
+        git log --pretty=format:'%h %ci %s' origin/main
+        echo ""
+        echo ""
+    fi
     git checkout -b main --track origin/main
 
     COMMIT_SHA=$(git rev-parse HEAD)