workflows/nixpkgs-vet: Make merge check script reusable

This is useful for other workflows as well. Originally I thought it
couldn't be put in the repo, but it can (just needs another checkout)

ci/get-merge-commit.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# This checks for mergeability of a pull request as recommended in
+# 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
+
+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")
+    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 1
+        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"
+    jq -r .merge_commit_sha <<< "$prInfo"
+else
+    log "The PR has a merge conflict"
+    exit 1
+fi