Trap syscall.SIGTERM instead of os.Kill

Fixed ineffectual assignments in tests

Simplify code

os.Kill can not be signal trapped. I assume we want to trap syscall.SIGTERM
instead?

Don't assign values we don't end up checking.

- Use bytes.Equal instead of bytes.Compare
- Use time.Since instead of manual timestamp subtracting
- Use sort.Strings instead of sort.StringSlice
- No need to specify the len of the slice

Allow hub used in Actions by specifying GITHUB_USER

When supplied with GITHUB_TOKEN, hub will attempt to request the `user`
API resource which isn't available in the context of Actions.

To work around this, scripts may now set GITHUB_USER to avoid hitting
the unavailable API resource.

Hub is impossible to use from within GitHub Actions because specifying
GITHUB_TOKEN will trigger the code path where the `user` resource is
requested from the GitHub API to get the current login name, but this
resource is not available for that token.

This failure message was cryptic and misleading before as it was
sometimes presented as HTTP 401. This change ensures that the correct
HTTP 403 "Resource not accessible by integration" message is shown.

Improve push target detection for `push.default=upstream`

When `git config push.default` is "upstream" or "tracking", but the
current branch is pushed to a remote without having upstream
configuration set up (for example, via `git push <REMOTE> HEAD` without
`-u`), this change makes it so that the remote tracking branch (the push
target) is still discovered via the same mechanism as if `push.default`
wasn't set (i.e. iterating through all the remotes).

https://github.com/github/hub/issues/189#issuecomment-507639770

`cmd.New(programWithSpaces)` will now work for executables that have
spaces in the path. The only place where we relied on shell-splitting
was in openTextEditor, which now performs its own splitting.

Avoids shell-splitting manProgram unless the value was read from the
$PAGER environment variable.

Makes possible:

    BROWSER='"/Applications/Google Chrome.app/Contents/MacOS/Google Chrome" --extra --flag' hub browse

Fixes #2178

Docs: mention that comma-separated lists must not have spaces

Fixes #2176

build: pass mod vendoring flag on newer versions of go

The go compiler doesn't like it when you mix the two, and Travis CI does
exactly that. Take advantage of the opportunity to disable it everywhere
else as well.

When go autodetects that it is being run as a go mod, and that there is
a vendor directory, it will still try to redownload all sources over the
network, unless you use -mod=vendor to tell it to use that. Additionally,
when using -mod=vendor the compiler will nicely avoid messing with
$GOPATH at all, since it can operate in a completely self-contained
manner.

Take advantage of this, when the detected go version is at least 1.11
(when the -mod flag was introduced).

Implement `hub api --paginate`

The GraphQL query has to accept the optional `endCursor` string variable
and output `pageInfo`:

    pageInfo {
      hasNextPage
      endCursor
    }

Repeated calls to `client.simpleApi()` will memoize its result and avoid
making multiple instances of `http.Client` and the associated Transport.

check head branch exists before creating PR

This is the heuristics we apply to determine if a branch was pushed:

1. The branch has remote tracking information (`branch.<branch>.remote`
   and `branch.<branch>.merge` config) that does NOT point to the
   base branch for this pull request; or

2. There exists a `refs/remotes/*/<branch>` ref in any git remote.

Otherwise, we consider the branch NOT pushed and we abort before
processing the pull-request command further.

[pr] add ability to open Pull Request in the browser

Closes #1586

Add a milestone name instead of an id, when creating 

Add `hub pull-request --no-maintainer-edits` flag

If `hub pr checkout <NUMBER>` was used, the tracking configuration for
the created branch might look like this for forks:

    branch.<branch>.remote https://github.com/<owner>/<repo>.git
    branch.<branch>.merge refs/heads/<branch>

Since `RemoteBranchAndProject()` doesn't handle this case, we can't use
it. I want to avoid expanding its functionality right now since it's
being used in several places and I don't want to risk causing
regressions in other commands. Instead, `pr show` now performs its own
upstream branch lookup.