Make `checkout` work independently of remote refspec

This fixes:

    fatal: Cannot setup tracking information; starting point 'BRANCH' is not a branch.

if the remote refspec is set up to only fetch specific branches and NOT
like the default `+refs/heads/*:refs/remotes/origin/*`.

I don't understand why would git, after fetching into
`refs/remotes/origin/BRANCH`, complain that `origin/BRANCH` is not a
branch, but there we have it. Setting up upstream tracking manually
works around this problem.

Add pull-request --no-edit flag to zsh completion

Documentation of install on Debian (sid/buster)

Squash on merge

Ref. #2043

Fixes #718 by documenting that the package is in the official repository now, or perhaps there is also a need for a package for the current stable Debian version.

List PR "Draft" state using the `--format` string

- `%pS`: "open", "draft", "merged", or "closed"
- `%pC`: green, gray, purple, or red

Have commands with rich output respect the `--color` flag

Add FreeBSD installation instructions

Make man pages parseable with `whatis`

- default: `--color=auto`
- `--color` is equivalent to `--color=always`
- `--color=never` disables color for TTYs

Outputs the "NAME" section in roff format to fix the following:

    mandb: warning: ... whatis parse for hub-alias(1) failed

Fixes https://github.com/github/hub/issues/2025

Fixes https://github.com/github/hub/issues/2036

Assume that 4xx responses will be the same given the same request, so
these responses should be subject to caching too.

Avoids caching 403 because that is the status returned when GitHub API
limit is hit.

Continue not caching 5xx responses because they indicate server errors,
so it's possible that a 2nd request of identical parameters succeeds.

Allow raw request body via `hub api --input <FILE>`

Fixes #2022

Consistent ordering and `--format` support for `ci-status`

Statuses are now sorted the same as in the GitHub web interface:

- failing first;
- then pending;
- then success/neutral;
- everything in-between is sorted alphabetically.

Fixes #2021

Add HTTP request/response headers functionality to `hub api`

Fix subcommand execution under WSL

Makefile: fix arguments to go, not matching what go expects

- HTTP response is now printed on stdout regardless of HTTP status
- No longer print an extra newline after HTTP response body
- No more `Error: HTTP {STATUS}` message on stderr
- hub exits with status 22 instead of 1