Support HTTPS_PROXY, NO_PROXY from environment

[docs] Add link to apt package manager

Add a link to the apt package manager from Debian man pages.

implement format for `pr show`

Fixes #2288

[ci skip]

Add completions for git fork

Now that there are 3 different flags all starting with the same two
dashes, the previous test case (that pressing Tab will expand to
--no-remote) will fail.

The previous test case has been replaced with one that expects a list of
completion options matching those provided for git fork.

Because it wouldn't make sense to set a name for a remote when you ask
it not to add a remote (or vice versa) these two flags are mutually
exclusive.

For lack of an idea for a more elegant solution, I've made it so that
whenever either of them matches, the other will also be dropped from the
available flags list.  This works perfectly for now but will probably
need to be revisited if any other such parameters are added in future.

The --remote-name and --org parameters to git fork expect arguments
which can be considered free text because the user has to suggest a
name, so we can't provide suggestions there.

If one of those two was the previous word then we clear completion
suggestions until a free-text word has been entered, allowing the user
to add their own input there.

Because this is an array of flags now, adding new flags is as simple as
adding more elements to the array and to the case statement.

Instead of adding more if statements I swapped the remote=yes variable
for a list of available flags which we drop elements from on each
completion.  This is based on the logic in git create above.

Fixes #2108.

If a git tag was annotated, assume that it contains full release notes
and make a public release rather than a draft one.

This invocation would fail:

    GITHUB_HOST=example.com hub api graphql ...

This is because hub would erroneously try to POST to
`example.com/api/v3/graphql` instead of `example.com/api/graphql`.

This applies a workaround that gets rid of the "v3" portion for requests
to "graphql" endpoint.

Fixes #2287

`go fmt` doesn't seem to respect `-mod=vendor`
https://github.com/golang/go/issues/27841

Instead try using `gofmt` which doesn't trigger downloading dependencies
in the first place. However, `gofmt` affects files under `vendor/`, so
we reset that before checking for changes.

Go's official proxy implementation doesn't allow any proxy configuration
from environment variables to be applied to requests to `localhost`.

Previously we only supported HTTP_PROXY because the proxy support in Go
was lacking in older versions. Now that there is
`golang.org/x/net/http/httpproxy`, we can rely on that instead to gain
consistent support for HTTPS_PROXY and NO_PROXY.

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