workflows/get-merge-commit: inline get-merge-commit.sh script as github-script

The reason this was a separate shell script was, that this would be
included in multiple workflows separately. But a while ago this had been
changed to a re-usable workflow, so we can just as well inline the
script.

This also allows us to use actions/github-script, which makes for a much
more readable script than the bash script before.

ci/README.md

@@ -40,46 +40,3 @@ Why not just build the tooling right from the PRs Nixpkgs version?
 - Because it makes the CI check very fast, since no Nix builds need to be done, even for mass rebuilds.
 - Because it improves security, since we don't have to build potentially untrusted code from PRs.
   The tool only needs a very minimal Nix evaluation at runtime, which can work with [readonly-mode](https://nixos.org/manual/nix/stable/command-ref/opt-common.html#opt-readonly-mode) and [restrict-eval](https://nixos.org/manual/nix/stable/command-ref/conf-file.html#conf-restrict-eval).
-
-## `get-merge-commit.sh GITHUB_REPO PR_NUMBER`
-
-Check whether a PR is mergeable and return the test merge commit as
-[computed by GitHub](https://docs.github.com/en/rest/guides/using-the-rest-api-to-interact-with-your-git-database?apiVersion=2022-11-28#checking-mergeability-of-pull-requests) and its parent.
-
-Arguments:
-- `GITHUB_REPO`: The repository of the PR, e.g. `NixOS/nixpkgs`
-- `PR_NUMBER`: The PR number, e.g. `1234`
-
-Exit codes:
-- 0: The PR can be merged, the hashes of the test merge commit and the target commit are returned on stdout
-- 1: The PR cannot be merged because it's not open anymore
-- 2: The PR cannot be merged because it has a merge conflict
-- 3: The merge commit isn't being computed, GitHub is likely having internal issues, unknown if the PR is mergeable
-
-### Usage
-
-This script is implemented as a reusable GitHub Actions workflow, and can be used as follows:
-
-```yaml
-on: pull_request_target
-
-# We need a token to query the API, but it doesn't need any special permissions
-permissions: {}
-
-jobs:
-  get-merge-commit:
-    # use the relative path of the get-merge-commit workflow yaml here
-    uses: ./.github/workflows/get-merge-commit.yml
-
-  build:
-    name: Build
-    runs-on: ubuntu-24.04
-    needs: get-merge-commit
-    steps:
-      - uses: actions/checkout@<VERSION>
-        # Add this to _all_ subsequent steps to skip them
-        if: needs.get-merge-commit.outputs.mergedSha
-        with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
-      - ...
-```

ci/get-merge-commit.sh

@@ -1,65 +0,0 @@
-#!/usr/bin/env bash
-# See ./README.md for docs
-
-set -euo pipefail
-
-log() {
-    echo "$@" >&2
-}
-
-if (( $# < 2 )); then
-    log "Usage: $0 GITHUB_REPO PR_NUMBER"
-    exit 99
-fi
-repo=$1
-prNumber=$2
-
-# Retry the API query this many times
-retryCount=5
-# Start with 5 seconds, but double every retry
-retryInterval=5
-
-while true; do
-    log "Checking whether the pull request can be merged"
-    prInfo=$(gh api \
-        -H "Accept: application/vnd.github+json" \
-        -H "X-GitHub-Api-Version: 2022-11-28" \
-        "/repos/$repo/pulls/$prNumber")
-
-    # Non-open PRs won't have their mergeability computed no matter what
-    state=$(jq -r .state <<< "$prInfo")
-    if [[ "$state" != open ]]; then
-        log "PR is not open anymore"
-        exit 1
-    fi
-
-    mergeable=$(jq -r .mergeable <<< "$prInfo")
-    if [[ "$mergeable" == "null" ]]; then
-        if (( retryCount == 0 )); then
-            log "Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com/"
-            exit 3
-        else
-            (( retryCount -= 1 )) || true
-
-            # null indicates that GitHub is still computing whether it's mergeable
-            # Wait a couple seconds before trying again
-            log "GitHub is still computing whether this PR can be merged, waiting $retryInterval seconds before trying again ($retryCount retries left)"
-            sleep "$retryInterval"
-
-            (( retryInterval *= 2 )) || true
-        fi
-    else
-        break
-    fi
-done
-
-if [[ "$mergeable" == "true" ]]; then
-    log "The PR can be merged"
-    mergedSha="$(jq -r .merge_commit_sha <<< "$prInfo")"
-    echo "mergedSha=$mergedSha"
-    targetSha="$(gh api "/repos/$repo/commits/$mergedSha" --jq '.parents[0].sha')"
-    echo "targetSha=$targetSha"
-else
-    log "The PR has a merge conflict"
-    exit 2
-fi