Add parents field to timeline events

[lyoung-confluent]

The committed events from the timeline API can have a parents field containing the parent commits:

    "parents": [
      {
        "sha": "29480ba555d614487fddba278d32d19102e86527",
        "url": "https://api.github.com/repos/google/go-github/git/commits/29480ba555d614487fddba278d32d19102e86527",
        "html_url": "https://github.com/google/go-github/commit/29480ba555d614487fddba278d32d19102e86527"
      }
    ],

This is also documented on the issue event payload: https://docs.github.com/en/developers/webhooks-and-events/events/issue-event-types#event-object-properties-7

[codecov]

Codecov Report

Merging #2566 (35caea3) into master (ddfa307) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2566   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files         125      125           
  Lines       10875    10875           
=======================================
  Hits        10656    10656           
  Misses        150      150           
  Partials       69       69           

Impacted Files	Coverage Δ
github/issues_timeline.go	100.00% <ø> (ø)

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[gmlewis]

Thank you, @lyoung-confluent !
LGTM.
Merging.