Document init script behavior with multiple master remotes

[EliahKagan]

Summary

• Add a comment to init-tests-after-clone.sh explaining what happens when master is locally absent but present on more than one remote: git checkout master -- refuses to pick one (even when all of the remote-tracking branches agree on a commit), and the script falls back to git checkout -b master, creating the branch at the current HEAD. The reflog built up by the subsequent git reset --hard HEAD~1 calls then traces HEAD's history instead of a remote master's. This is harmless because master is reset to __testing_point__ afterward, but it is unintuitive enough to be worth noting in the script itself.

Closes #2145.

Test plan

• sh -n init-tests-after-clone.sh passes
• shellcheck init-tests-after-clone.sh passes
• Diff is comment-only; no behavior change

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds in-script documentation clarifying an unintuitive git checkout master -- || git checkout -b master edge case when multiple remotes provide a master branch, so future contributors understand why master’s reflog may end up reflecting HEAD history during test initialization (Closes #2145).

Changes:

• Document how git checkout master -- can fail due to ambiguous remote-tracking branches when master is missing locally and multiple remotes have it.
• Explain that the script then creates master at the current HEAD, affecting which history the reflog-reset sequence records (before resetting back to __testing_point__).

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[EliahKagan]

I didn't think of a better way to deal with this besides adding a comment. Also, I had originally thought of including this as part of some larger PR, but it doesn't really fit nicely in with any of the other forthcoming GitPython PRs I'm working on. So I'm doing it by itself. I think this at least better than not expanding the comment, and I plan to merge this once CI passes.

[EliahKagan]

Incidentally, I decided to try to have Claude Code proceed in its default way, rather than articulating a plan first. Overall, this is faster but kind of rough: for example, the branch name is meaningless, which is something I try to avoid. I don't think the branch name is worth the noise and churn of remaking the PR, though.