Add password successful feedback

This adds some feedback when entering the password on the command line.
When the password is entered and supplied through stdin (and stdout is a
terminal) then the a message saying `password is correct` if correct is
printed.

cmd/restic/global.go

@@ -132,6 +132,20 @@ func restoreTerminal() {
 	})
 }
 
+// shouldOutputPasswordSuccessful returns true if the password is supplied
+// interactively and the stdout is a terminal.
+func shouldOutputPasswordSuccessful() bool {
+	if globalOptions.PasswordFile != "" {
+		return false
+	}
+
+	if os.Getenv("RESTIC_PASSWORD") != "" {
+		return false
+	}
+
+	return stdoutIsTerminal()
+}
+
 // ClearLine creates a platform dependent string to clear the current
 // line, so it can be overwritten. ANSI sequences are not supported on
 // current windows cmd shell.
@@ -326,6 +340,10 @@ func OpenRepository(opts GlobalOptions) (*repository.Repository, error) {
 		return nil, err
 	}
 
+	if shouldOutputPasswordSuccessful() {
+		Verbosef("password is correct\n")
+	}
+
 	if opts.NoCache {
 		return s, nil
 	}