Support auth token _get_commit_msg

[anish]

#3443 introduced the _get_commit_msg function, that interacts with the github api. This breaks the use of github event hook on any GitHub instances that enforce authentication. Add support for auth token

[tardyp]

looks good. need to fix unit tests.

[anish]

@tardyp working on it, I'm seeing 50 odd unrelated failures locally, need to fix the venv first

[codecov]

Codecov Report

Merging #3683 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3683      +/-   ##
==========================================
+ Coverage   88.44%   88.44%   +<.01%     
==========================================
  Files         323      323              
  Lines       33967    33976       +9     
==========================================
+ Hits        30043    30051       +8     
- Misses       3924     3925       +1

Impacted Files	Coverage Δ
master/buildbot/www/hooks/github.py	95.23% <100%> (+0.11%)	⬆️
master/buildbot/worker/docker.py	79.48% <0%> (-0.52%)	⬇️
master/buildbot/process/builder.py	84.44% <0%> (-0.11%)	⬇️
master/buildbot/steps/shell.py	97.45% <0%> (ø)	⬆️
master/buildbot/worker/hyper.py	89.38% <0%> (+0.88%)	⬆️

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[anish]

@tardyp all done

[tardyp]

great job! thanks!