Describe the bug

Bash shell completions should add a space after completing a command name.

Steps to reproduce the behavior

0. If not already, set up completions according to gh help completion
1. Type gh pr ch
2. Press Tab

Expected vs actual behavior

The word ch expands to checkout without a space after, but I think users would prefer to have a space automatically added after the word so they can easily add an argument. For comparison, git com Tab expands to commit with a space added after.

Ref. spf13/cobra#371