[BUGFIX] TSDB: Don't read in-order chunks from before head MinTime

Because we are reimplementing the `IndexReader` to fetch in-order and
out-of-order chunks together, we must reproduce the behaviour of
`Head.indexRange()`, which floors the minimum time queried at `head.MinTime()`.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/db.go

@@ -2060,7 +2060,7 @@ func (db *DB) Querier(mint, maxt int64) (_ storage.Querier, err error) {
 
 	overlapsOOO := overlapsClosedInterval(mint, maxt, db.head.MinOOOTime(), db.head.MaxOOOTime())
 	var headQuerier storage.Querier
-	inoMint := mint
+	inoMint := max(db.head.MinTime(), mint)
 	if maxt >= db.head.MinTime() || overlapsOOO {
 		rh := NewRangeHead(db.head, mint, maxt)
 		var err error
@@ -2138,7 +2138,7 @@ func (db *DB) blockChunkQuerierForRange(mint, maxt int64) (_ []storage.ChunkQuer
 
 	overlapsOOO := overlapsClosedInterval(mint, maxt, db.head.MinOOOTime(), db.head.MaxOOOTime())
 	var headQuerier storage.ChunkQuerier
-	inoMint := mint
+	inoMint := max(db.head.MinTime(), mint)
 	if maxt >= db.head.MinTime() || overlapsOOO {
 		rh := NewRangeHead(db.head, mint, maxt)
 		headQuerier, err = db.blockChunkQuerierFunc(rh, mint, maxt)