CI: Several `read_only_api_tests` are broken on master

[DilumAluthge]

Repositories: Test Failed at /home/runner/work/GitHub.jl/GitHub.jl/test/read_only_api_tests.jl:89
  Expression: (compare(ghjl, "master", "master~"; auth = auth)).behind_by == 1
   Evaluated: 7 == 1

Repositories: Test Failed at /home/runner/work/GitHub.jl/GitHub.jl/test/read_only_api_tests.jl:91
  Expression: comparison.ahead_by == 1
   Evaluated: 7 == 1

Repositories: Test Failed at /home/runner/work/GitHub.jl/GitHub.jl/test/read_only_api_tests.jl:92
  Expression: length(comparison.commits) == 1
   Evaluated: 7 == 1

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Example PR:

• Trigger CI #226

Example CI log: https://github.com/JuliaWeb/GitHub.jl/actions/runs/15786277745/job/44503269205?pr=226

[tanmaykm]

These tests seem to assume (incorrectly) that the master branch will not have a merge commit. I think we need to have a conditional check here. Will add a PR to update the tests.