Fix 2.1.0 regression

[coilysiren]

Motivation

Fixes #1015

Release Notes

Fixed a Context regression introduced in #1013

Changes

When passing in context.Background(), initialize a *flagSet value. This prevents the code from panicking when flagSet is accessed later.

Testing

In addition to the automated tests, I stood up a testing package (here => coilysiren/testing-cli#1) with this new code. The testing package ran successfully with this new cli code.

Followup

After v2.1.1 is released, I'll remove the commented out documentation lines. Also, this issue creates more justification for investigating #1006.

Reviewer Guidelines

All our builds are broken until this is resolved 😅

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ed1a2ab). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #1014   +/-   ##
========================================
  Coverage          ?   72.5%           
========================================
  Files             ?      33           
  Lines             ?    2495           
  Branches          ?       0           
========================================
  Hits              ?    1809           
  Misses            ?     575           
  Partials          ?     111

Impacted Files	Coverage Δ
context.go	95.45% <100%> (ø)

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[coilysiren]

In case it'd be helpful to identify, the cause here is this diff

- context := NewContext(a, set, nil)
+ context := NewContext(a, set, &Context{Context: ctx})

via https://github.com/urfave/cli/pull/975/files#diff-a66cc59eb6bfe62b7b3b69cd52b84747L227-R235

[asahasrabuddhe]

LGTM