[sparrowren]

Both contribution READMEs tell contributors to merge upstream/main, but this repository has no main branch. A fresh clone of a contributor's fork also does not have an upstream remote configured.

Update both READMEs to explain the one-time upstream setup and selecting the branch the PR will target. Use <target-branch> throughout, with instructions to replace it with a real branch name such as master or v5.6. If the branch does not exist locally, explicitly create it tracking origin/<target-branch> so Git does not have to choose between matching branches on both remotes. Synchronize with upstream/<target-branch> using a fast-forward merge, then push to the fork. Explain saving local changes and resolving divergent histories before pushing.

Validation:

• Reproduced the original missing-upstream and missing-main errors.
• Ran the command blocks from both READMEs against local bare upstream/fork repositories: eight scenarios covering both languages, master and v5.6, and existing and missing local target branches. Each local branch and fork reached the incoming upstream commit.
• Reproduced the ambiguous git switch error when the local target branch is absent and both remotes contain it; verified the explicit creation command tracks origin/<target-branch>.
• Confirmed that --ff-only rejects divergent histories and preserves both the local commit and the fork in all eight scenarios.
• git diff --check passed.

Initial error reproduction ran with Git for Windows. The expanded target-branch validation ran in WSL Ubuntu with Git 2.43.0. All validation pushes targeted local fixture repositories.

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[jiaoshuntian]

Thanks for fixing the synchronization instructions and updating both READMEs.

Could we use <target-branch> instead of hardcoding v5.6? Both master and v5.6 are maintained, so the instructions should explain that contributors need to select the branch their PR will target:

git fetch upstream
git switch <target-branch>
git merge --ff-only upstream/<target-branch>
git push origin <target-branch>

Please clarify that <target-branch> must be replaced with the actual branch name, such as master or v5.6.

Also, if the target branch does not exist locally, git switch <target-branch> can fail because both remotes contain a matching branch. Please document explicitly creating it with git switch -c <target-branch> --track origin/<target-branch> in that case, and keep both language versions aligned.

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