grep: make -l the default if no queries were provided

grep.go

@@ -146,9 +146,12 @@ func runGrep(args []string) int {
 	}
 	w := c.Walker(cflt)
 
-	// no need to actually grep if no queries were provided and only filenames were requested
-	// simple cache walk is enough and also will output results ordered by builder/origin/timestamp
-	if len(opts.Args()) == 0 && filenamesOnly {
+	// list only log filenames if no queries were provided
+	if len(opts.Args()) == 0 {
+		filenamesOnly = true
+
+		// no need to actually grep if no queries were provided and only filenames were requested
+		// simple cache walk is enough and also will output results ordered by builder/origin/timestamp
 		err = w.Walk(func(entry cache.Entry, err error) error {
 			if err != nil {
 				return err

main.go

@@ -11,10 +11,6 @@ import (
 	"github.com/dmgk/getopt"
 )
 
-const (
-	dateFormat = "2006-01-02"
-)
-
 var usageTmpl = template.Must(template.New("usage").Parse(`
 usage: {{.progname}} [-hV] [-M mode] [-G colors] command [options]
 
@@ -43,6 +39,7 @@ var (
 )
 
 const (
+	dateFormat      = "2006-01-02"
 	colorModeAuto   = "auto"
 	colorModeAlways = "always"
 	colorModeNever  = "never"