Include --help and --version flag in completion (#1813)

Fixes #1786

The --help, -h, --version and -v flags are normally added when the
`execute()` function is called on a command.  When doing completion
we don't call `execute()` so we need to add these flags explicitly to
the command being completed.

Also, we disable all further completions if the 'help' or 'version'
flags are present on the command-line.

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>

command.go

@@ -30,6 +30,8 @@ import (
 	flag "github.com/spf13/pflag"
 )
 
+const FlagSetByCobraAnnotation = "cobra_annotation_flag_set_by_cobra"
+
 // FParseErrWhitelist configures Flag parse errors to be ignored
 type FParseErrWhitelist flag.ParseErrorsWhitelist
 
@@ -1064,6 +1066,7 @@ func (c *Command) InitDefaultHelpFlag() {
 			usage += c.Name()
 		}
 		c.Flags().BoolP("help", "h", false, usage)
+		_ = c.Flags().SetAnnotation("help", FlagSetByCobraAnnotation, []string{"true"})
 	}
 }
 
@@ -1089,6 +1092,7 @@ func (c *Command) InitDefaultVersionFlag() {
 		} else {
 			c.Flags().Bool("version", false, usage)
 		}
+		_ = c.Flags().SetAnnotation("version", FlagSetByCobraAnnotation, []string{"true"})
 	}
 }