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.

Previously hub checked that there were
[no unpushed commits](https://github.com/github/hub/blob/b3b3ebb8e517acd269489b9bd163de2f8fd309fc/commands/pull_request.go#L219-L223)
but this check doesn't work when there is not even a remote branch.

This fixes #1930.

Fixes #2162

Move commitLogs declaration inside multiple commits block

Ignore VSCode

This makes it clearer that commits are only added as a commented section
in the case when there are multiple commits on a branch.

Update x/sys module to support Risc-V

Fixes #2146

migrate to go modules from dep

Enable GIT_TRACE

This was kept for backwards compatibility, but was silently broken for
some tiem now. Nobody reported it, so I think it can be safely assumes
that nobody uses this config value anymore.

For stderr, either attach it to parent stderr or silence it completely.
This enables GIT_TRACE.

Added Chocolatey as Windows install method in the readme

Propagate global git arguments to Before/After chains

This fixes `hub -C mydir merge <URL>` and other commands that might be
invoked with git global arguments by ensuring that those global
arguments are also fowarded to any accompanying `git` commands within
`Before()` and `After()` chains.