Improve `compare` upstream detection & documentation

Ref. #1697

- Correctly detect when the current branch is pushed to a fork
- Support upstream configuration with full remote URL
- More precise errors instead of just printing usage synopsis:
  - no current branch
  - the current branch doesn't seem pushed to a remote
  - the current branch is the same as the default branch
  - `--base` value is the same as current branch

It was wrong to just assume that stdin is "0" everywhere.

Followup to 7514fa2b

Fixes #2276

Works around the fact that `syscall.Stdin` doesn't seem to be of the
same type (int) across Windows and other systems:

    $ GOOS=windows GOARCH=amd64 make
    script/build -o bin/hub
    # github.com/github/hub/github
    github/config.go:196:44: cannot use stdin (type syscall.Handle) as type int in argument to terminal.GetState
    github/config.go:205:19: cannot use stdin (type syscall.Handle) as type int in argument to terminal.Restore
    github/config.go:215:41: cannot use stdin (type syscall.Handle) as type int in argument to terminal.ReadPassword
    make: *** [bin/hub] Error 2

https://travis-ci.org/github/hub/jobs/587860665

Fix parsing `~/.ssh/config` on Windows

Thank you @VladimirUAZ for helping debug this! ❤️ 

Use `homedir.Dir()` instead of `Getenv("HOME")` on Windows to look up
`~/.ssh/config` in the correct location.

Added `hub pr` fish completions and wrapped `git` completions

* Redefined `__fish_hub_using_command` to support multiple commands (such as `hub pr list`)
* Added `hub pr list` and `hub pr checkout` along with their options

Make hub easier to use from GitHub Actions

This allows hub to be easier used within GitHub Actions where GITHUB_REPOSITORY is pre-populated by default. If GITHUB_USER is not set in the environment, its value is inferred from the part of GITHUB_REPOSITORY value until the first slash character.

[ci skip]

hub api url generation fixes

Update Cucumber and Aruba test runners

This replaces Aruba's usage of ChildProcess with `popen3`. It supports
no process timeouts, but damn it's fast.

https://travis-ci.org/github/hub/jobs/574964882