hub: Update to 2.8.3

pkgsrc changes:
* Add logic to generate man page via pre-install target instead of
  generating it manually

Changes:
### Changes since v2.7
* New `hub api` command for scripting with GitHub API
  <https://github.com/github/hub/pull/2016>
* Re-implement CLI flag parsing so that `--message <MSG>` is equivalent to
  `--message=<MSG>` <https://github.com/github/hub/pull/2008>
* Re-implement `make man-pages` in Go instead of Ruby
  <https://github.com/github/hub/pull/1990>
* `issue create --label` is now `issue create --labels` to align with
  existing documentation
* Output crash debugging information on stderr instead of stdout
* Build improvements:
  - respect environment LDFLAGS
  - strip the build path from resulting executable
  - enable reproducible builds with SOURCE_DATE_EPOCH

(leot)