cmd/zoekt-indexserver: log message on deleting stale shard

Change-Id: I9ac29cfabb17e014555c56ff192218f768bd8d49
diff --git a/cmd/zoekt-indexserver/main.go b/cmd/zoekt-indexserver/main.go
index 7431ac2..381407a 100644
--- a/cmd/zoekt-indexserver/main.go
+++ b/cmd/zoekt-indexserver/main.go
@@ -203,6 +203,7 @@
 
 	_, err = os.Stat(gitindex.Path(repoDir, u))
 	if os.IsNotExist(err) {
+		log.Printf("deleting stale shard %s for %q", fn, u)
 		return os.Remove(fn)
 	}