Ignore required flags when DisableFlagParsing (#1095)

When a command request to DisableFlagParsing, it should not fail due
to a missing required flag.  In fact, such a check will always fail
since flags weren't parsed!

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>

command.go

@@ -979,6 +979,10 @@ func (c *Command) ValidateArgs(args []string) error {
 }
 
 func (c *Command) validateRequiredFlags() error {
+	if c.DisableFlagParsing {
+		return nil
+	}
+
 	flags := c.Flags()
 	missingFlagNames := []string{}
 	flags.VisitAll(func(pflag *flag.Flag) {