hub 2.13.0

* New command: `hub gist`

* Add `git pr show --format=...` flag

* Add `hub api --obey-ratelimit` flag

* `hub help`: support compressed man pages

* Add `hub release download --include=PATTERN` flag

* `hub help`: Enable `hub help --web <command>`

* `hub release`: Retry attaching release assets on 5xx server errors

* `hub pr checkout` fixes
  - handle case when the PR given matches the current branch
  - avoid overriding existing branch merge config

* `hub release`: Fail fast when attempting to attach unavailable files

* Honor HTTPS_PROXY, NO_PROXY environment variables

* Strip "co-authored-by" lines when populating PR body from git log

* git compatibility: have `hub <cmd> --help` open the man page

* `hub fork` bash completion improvements

* `hub pr` fish completion improvements