The over-all grand plan of the Azure Pipelines -> GitHub Actions migration

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

pipelines-to-github-actions-gameplan.md

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+## Workflows to add
+
+- handle-pr-comment
+  - **replaces**:
+    1. [GitGitGadget PR Handler](https://dev.azure.com/gitgitgadget/git/_build?definitionId=3)
+    2. [GitGitGadget PR Handler (git)](https://dev.azure.com/gitgitgadget/git/_build?definitionId=13)
+    3. [GitGitGadget PR Handler (dscho)](https://dev.azure.com/gitgitgadget/git/_build?definitionId=12)
+  - **triggers on**: PR comments by allowed users
+  - **needs**: SMTP credentials
+- handle-pr-push
+  - **replaces**:
+    1. [GitGitGadget PR Handler](https://dev.azure.com/gitgitgadget/git/_build?definitionId=3)
+    2. [GitGitGadget PR Handler (git)](https://dev.azure.com/gitgitgadget/git/_build?definitionId=13)
+    3. [GitGitGadget PR Handler (dscho)](https://dev.azure.com/gitgitgadget/git/_build?definitionId=12)
+  - **triggers on**: PR pushes
+- handle-new-mails
+  - **replaces**:
+    1. [Mirror Git List to GitGitGadget's PRs](https://dev.azure.com/gitgitgadget/git/_build?definitionId=5)
+  - **triggers on**: `git-mailing-list-mirror` updates
+  - **needs**: git-mailing-list checkout
+- update-prs
+  - **replaces**:
+    1. [Update GitGitGadget's PRs](https://dev.azure.com/gitgitgadget/git/_build/index?definitionId=2)
+  - **triggers on**: `seen` updates
+  - runs: updateOpenPrs(), updateCommitMappings() and handleOpenPRs()
+- update-mail-to-commit-notes
+  - **replaces**:
+    1. [Update GitGitGadget's commit to mail notes](https://dev.azure.com/gitgitgadget/git/_build?definitionId=9)
+  - **triggers on**: `seen` updates
+  - TODO: convert from using `gitster/seen` to `upstream/seen`
+  - **needs**: git-mailing-list checkout
+  - **runs**:
+    ```
+    rev="$(git -C "$GITGIT_DIR" rev-parse refs/notes/commit-to-mail)"
+    ./gitgitgadget/script/lookup-commit.sh --notes update
+    if test "$rev" != "$(git -C "$GITGIT_DIR" rev-parse refs/notes/commit-to-mail)"
+    then
+      ./gitgitgadget/script/update-mail-to-commit-notes.sh
+
+      git -C "$GITGIT_DIR" push https://git-for-windows-ci:$(git-for-windows-ci.github.token)@github.com/gitgitgadget/git refs/notes/commit-to-mail refs/notes/mail-to-commit ||
+      die "Could not push notes"
+
+      no_match="$(git -C "$GITGIT_DIR"/ show refs/notes/commit-to-mail | grep -B2 no\ match | sed -ne 's/\///g' -e 's/^\+\+\+ b//p')"
+      test -z "$no_match" ||
+      die "Could not find mail(s) for: $no_match"
+    fi
+    ```