Fix bash-completion for git 1.7.2.

The completion for git 1.7.2 parses the output of `git help --all`
to generate a list of available commands. The output from `hub help`
does not conform to the same format or respect the `--all` flag.

To fix this issue, we only display hub's extended help when the
`--all` (or `-a`) flag is not present, falling back to git.

lib/hub/commands.rb

@@ -478,7 +478,7 @@ module Hub
       if command == 'hub'
         puts hub_manpage
         exit
-      elsif command.nil?
+      elsif command.nil? && args.grep(/^--?a/).empty?
         ENV['GIT_PAGER'] = '' if args.grep(/^-{1,2}p/).empty? # Use `cat`.
         puts improved_help_text
         exit