Log Message:
hub: Update to 2.12.0

Changes:
2.12.0
------
## Features
* Add `hub pr show [<number>]` command to open a pull request in the browser
* Add `hub api --paginate` option to automatically fetch all pages of results
* Add `hub pull-request --no-maintainer-edits` flag to disallow project
  maintainers being able to push to your branch
* `hub issue -M` can now accept milestone names instead of just numbers
* Abort `hub pull-request` with a helpful error message if the current branch
  wasn't pushed to a git remote
* Drop support for legacy `hub.http-clone` git config value

## Fixes
* Fix querying git configuration when GIT_TRACE is used
* Support detached HEAD if `hub pull-request --head` was passed
* Fix newline in `hub create` error message
* Ensure HTTP connection reuse across API calls