command: backport patch for mitchellh/cli help flag handling (#3536)

This patch backports a fix which will show the correct usage screen for
command line flags.

This is considered a temporary fix until the code has been refactored.
Newer versions of the cli library require that the flag set is populated
when Help() is called or that it is populated within Help() itself.

Fixes #3536

vendor/github.com/mitchellh/cli/cli.go

@@ -578,12 +578,6 @@ func (c *CLI) processArgs() {
 			break
 		}
 
-		// Check for help flags.
-		if arg == "-h" || arg == "-help" || arg == "--help" {
-			c.isHelp = true
-			continue
-		}
-
 		// Check for autocomplete flags
 		if c.Autocomplete {
 			if arg == "-"+c.AutocompleteInstall || arg == "--"+c.AutocompleteInstall {
@@ -604,6 +598,12 @@ func (c *CLI) processArgs() {
 				continue
 			}
 
+			// Check for help flags.
+			if arg == "-h" || arg == "-help" || arg == "--help" {
+				c.isHelp = true
+				continue
+			}
+
 			if arg != "" && arg[0] == '-' {
 				// Record the arg...
 				c.topFlags = append(c.topFlags, arg)