cli: Teach oc image mirror to accept a file from stdin

`oc image mirror -f -` should accept file definitions from STDIN like
the rest of our commands.

Author: smarterclayton

Diff for: pkg/oc/cli/image/mirror/mappings.go

@@ -3,6 +3,7 @@ package mirror
 import (
 	"bufio"
 	"fmt"
+	"io"
 	"os"
 	"strings"
 	"sync"
@@ -104,14 +105,17 @@ func parseArgs(args []string, overlap map[string]string) ([]Mapping, error) {
 	return mappings, nil
 }
 
-func parseFile(filename string, overlap map[string]string) ([]Mapping, error) {
+func parseFile(filename string, overlap map[string]string, in io.Reader) ([]Mapping, error) {
 	var fileMappings []Mapping
-	f, err := os.Open(filename)
-	if err != nil {
-		return nil, err
+	if filename != "-" {
+		f, err := os.Open(filename)
+		if err != nil {
+			return nil, err
+		}
+		defer f.Close()
+		in = f
 	}
-	defer f.Close()
-	s := bufio.NewScanner(f)
+	s := bufio.NewScanner(in)
 	lineNumber := 0
 	for s.Scan() {
 		line := s.Text()

Diff for: pkg/oc/cli/image/mirror/mirror.go

@@ -151,7 +151,7 @@ func (o *MirrorImageOptions) Complete(cmd *cobra.Command, args []string) error {
 		return err
 	}
 	for _, filename := range o.Filenames {
-		mappings, err := parseFile(filename, overlap)
+		mappings, err := parseFile(filename, overlap, o.In)
 		if err != nil {
 			return err
 		}