The previous format of documentation made it look like `--message VALUE`
(no equal sign) would be supported, which might be misleading.

This queries by `state=closed` and selects only PRs that have a merged date.

Fixes #1913

The default direction when sorting by `created` date (the default) is
descending. However, the default on the API backend changes to ascending
when the sort key is something different than `created`:
https://developer.github.com/v3/pulls/#list-pull-requests

This ensures that the default `direction: "desc"` is always passed to
API queries, regardless of the sort key, unless `--sort-ascending` was used.

Ref. https://github.com/github/hub/issues/1815#issuecomment-433222344

Previously, when `:` wouldn't appear in the value for `head`, the GitHub
API would ignore the `head` parameter altogether. Now, ensure that
`<OWNER>:` syntax is always populated by adding the default from the
current repository.

We need to be in a git repo for the tests to work.

This is the only way I know how to get info out from hub while running
the cucumber feature tests.

Should be removed before submitting as a PR.

It prints the help text.

"hub pr" still untested.

Help still untested.

WARNINGS:
* Entirely untested
* "hub pr" currently works as a synonym for "hub checkout", it should
do something else

The current functionality should really be moved to "hub pr checkout".
This is a step on the way.

It wraps functionality already provided by "hub checkout". The advantage
with "hub review" are twofold:

* Discoverability. It wasn't until I started browsing the hub source
  code that I realized this functionality was present.
* Usability. Just typing the PR number is easier than having to figure
  out its URL.