add migrate-pr.sh script to help migration

tools/absorb-repo/README.md

@@ -14,3 +14,23 @@ The above invocation will create one or two commits on the `absorb-worker` branc
 - a merge commit that merges the `absorb-worker` branch with the rewritten default branch of `git@github.com:codecov/worker.git`
 
 You can run it multiple times to absorb multiple repositories in a single branch.
+
+### Migrating existing PRs
+
+Pull requests that weren't ready to be merged before the cutover can be migrated to `umbrella` using `migrate-pr.sh`. In order to use it, you need to have `gh` and `jq` installed. The simplest way to get them is with homebrew:
+```
+# Assuming you already have homebrew
+$ brew install gh jq
+
+# Log into the GH CLI
+$ gh auth login
+```
+
+Script usage:
+```
+./migrate-pr.sh https://github.com/codecov/worker/pull/1300
+```
+
+The above invocation will:
+- apply the same transformation to the PR's branch that `absorb-repo.sh` does
+- create a PR with the same title and body (with a link to the original PR) for the transformed branch

tools/absorb-repo/migrate-pr.sh

@@ -0,0 +1,67 @@
+#!/bin/bash
+
+pull_request=$1
+
+
+_jq_head_repo_owner=".headRepositoryOwner = .headRepositoryOwner.login"
+_jq_head_repo_name=".headRepository = .headRepository.name"
+
+__jq_reviewed_by="with_entries(if .key == \"reviews\" then .value = [.value[].author.login] else . end)"
+__jq_review_requested_from="with_entries(if .key == \"reviewRequests\" then .value = [.value[].slug] end)"
+_jq_reviewers="$__jq_reviewed_by | $__jq_review_requested_from | .reviewers = ([.reviews, .reviewRequests] | flatten) | del(.reviews) | del(.reviewRequests)"
+
+jq_query="$_jq_head_repo_owner | $_jq_head_repo_name | $_jq_reviewers"
+
+pr_data=$(gh pr view $pull_request --json title,body,headRefName,headRepositoryOwner,headRepository,reviews,reviewRequests | jq "$jq_query")
+
+pr_title=$(echo $pr_data | jq -r '.title')
+pr_body=$(echo $pr_data | jq -r \""(migrated from $pull_request)\r\n\r\n\""' + .body')
+pr_head_repo=$(echo $pr_data | jq -r '.headRepository')
+pr_head_repo_owner=$(echo $pr_data | jq -r '.headRepositoryOwner')
+pr_head_ref=$(echo $pr_data | jq -r '.headRefName')
+pr_reviewers=$(echo $pr_data | jq -r '.reviewers' | sed 's/\[/-r /' | sed 's/,/-r /g' | sed 's/\]//')
+
+# Assumes this script's directory has a sibling directory called `git-filter-repo`
+# which contains a copy of the `git-filter-repo` script.
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+GIT_FILTER_REPO_DIR="$(realpath "$SCRIPT_DIR/../git-filter-repo")"
+
+current_branch="$(git rev-parse --abbrev-ref HEAD)"
+migrated_ref="$pr_head_repo/$pr_head_ref"
+remote_name="$pr_head_repo-orig"
+
+case $pr_head_repo in
+    "worker" | "codecov-api")
+        subdirectory="apps/$pr_head_repo"
+        ;;
+    "shared")
+        subdirectory="libs/$pr_head_repo"
+        ;;
+    *)
+        echo "Uh oh"
+        exit 1
+        ;;
+esac
+
+if ! $(git remote | grep $remote_name); then
+    echo "Adding remote for $pr_head_repo_owner/$pr_head_repo..."
+    git remote add $remote_name git@github.com:$pr_head_repo_owner/$pr_head_repo || true
+    echo "Done"
+fi
+
+echo "Checking out $pr_head_repo/$pr_head_ref as _$migrated_ref..."
+git fetch $remote_name $pr_head_ref
+git checkout -b "_$migrated_ref" $remote_name/$pr_head_ref
+
+# Going back to the starting branch to run branch mutation
+git checkout "$current_branch"
+python "$GIT_FILTER_REPO_DIR/git-filter-repo" --force --refs "_$migrated_ref" --to-subdirectory-filter "$subdirectory"
+
+# Create a branch in this repository to merge into
+git checkout -b $migrated_ref
+git merge "_$migrated_ref" --allow-unrelated-histories --no-edit
+git branch -D _$migrated_ref
+git remote remove $remote_name
+
+git push origin $migrated_ref
+gh pr create --title "$pr_title" --body "$pr_body" $pr_reviewers