cmd/age: decide to buffer output based on stdin source

Buffering only when the armorFlag is set disregards use cases where data
from a tty stdin is decrypted or where binary data goes to a tty stdout.

Buffering is only necessary if stdin is a tty and stdout is a tty.

Co-authored-by: Filippo Valsorda <hi@filippo.io>

cmd/age/age.go

@@ -176,13 +176,6 @@ func main() {
 		defer f.Close()
 		out = f
 	} else if terminal.IsTerminal(int(os.Stdout.Fd())) {
-		if armorFlag {
-			// If the output will go to a TTY, and it will be armored, buffer it
-			// up so it doesn't get in the way of typing the input.
-			buf := &bytes.Buffer{}
-			defer func() { io.Copy(os.Stdout, buf) }()
-			out = buf
-		}
 		if name != "-" {
 			if decryptFlag {
 				// TODO: buffer the output and check it's printable.
@@ -193,6 +186,13 @@ func main() {
 					`Did you mean to use -a/--armor? Force with "-o -".`)
 			}
 		}
+		if in == os.Stdin && terminal.IsTerminal(int(os.Stdin.Fd())) {
+			// If the input comes from a TTY and output will go to a TTY,
+			// buffer it up so it doesn't get in the way of typing the input.
+			buf := &bytes.Buffer{}
+			defer func() { io.Copy(os.Stdout, buf) }()
+			out = buf
+		}
 	}
 
 	switch {