Merging existing repos into a monorepo

Note

The below steps have been automated by the tool in https://github.com/mosuem/merger.

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(Instructions in-lined from https://github.com/dart-lang/tools/pull/100.)

Steps taken for a sample graphs repo and target tools monorepo:

1. Get git-filter-repo:
   newren $ git clone git@github.com:newren/git-filter-repo.git
2. Add git-filter-repo to PATH.
3. Fresh clone of repo that needs to be merged into a mono repo:
   dart-lang $ git clone git@github.com:dart-lang/graphs.git
4. Destructively rewrite the history of the repository to be in pkgs/package_name:
   graphs $ git filter-repo --path-rename :pkgs/graphs/
5. Destructively prefix all the tags:
   graphs $ git filter-repo --tag-rename :graphs-
6. Replace paths and links in files.
   1. Badges: https://github.com/dart-lang/graphs/actions/workflows/ci.yml -> https://github.com/dart-lang/tools/actions/workflows/graphs.yml
   2. Remaining links, manually check if this does the right thing, otherwise insert steps above this step. https://github.com/dart-lang/graphs -> https://github.com/dart-lang/tools/tree/main/pkgs/graphs
7. Migrate the GitHub workflow.
   1. Delete dependabot.yaml which is already in the target mono_repo.
   2. Rename file: pkgs/graphs/.github/workflows/ci.yml -> .github/workflows/graphs.yml
   3. Add paths to the on clauses in the GitHub workflow (example).
   4. Add defaults->run->working-directory to all jobs in the GitHub workflow (example).
8. Commit with a detailed message.
9. In the directory of the mono_repo, create a branch.
   tools $ git checkout -b merge-graphs-package
10. Add a remote for the local clone of the moving package
    1. tools $ git remote add graphs_package /Users/dacoharkes/src/dart-lang/graphs/
    2. tools $ git fetch graphs_package
11. Merge tools $ git merge --allow-unrelated-histories graphs_package/master -m "Merge package:graphs into shared tool repository"
12. Copy the list of renamed tags into a comment on the PR.
13. Create a pull request and get it reviewed. Do not submit through the github UI, since it does not allow a fast forward merge.
    1. If there are changes required, back out and start over (it's OK to force push to the PR branch)
14. Disable squash-only in GitHub settings, and merge with a fast forward merge to the main branch, enable squash-only in GitHub settings.
15. Push tags to github
16. Follow up with a PR adding links to the top-level readme table.
17. Add a commit to https://github.com/dart-lang/graphs/ with it's readme pointing to this repo
18. Archive https://github.com/dart-lang/graphs/.