Change log field from `hash` to `commit`

`hash` is not really specific.

main.go

@@ -135,7 +135,7 @@ func main() {
 	logger = level.NewFilter(logger, filter)
 
 	level.Info(logger).
-		Log("hash", commitHash, "build", buildTime)
+		Log("commit", commitHash, "build", buildTime)
 
 	db, err := openDB(*dbPath, logger)
 	if err != nil {