feat: try to read ref and sha from event payload if available

With this change `act` will try to populate the `githubContext.ref` and
`githubContext.sha` with values read from the event payload.

Caveats:
- `page_build` should not have a ref
- `status` should not have a ref
- `registry_package` should set the ref to the branch/tag but the
  payload isn't documented
- `workflow_call` should set ref to the same value as its caller but the
  payload isn't documented
- most of the events should set the sha to the last commit on the ref
  but unfortunately the sha is not always included in the payload,
  therefore we use the sha from the local git checkout

pkg/runner/run_context.go

@@ -576,25 +576,47 @@ func (rc *RunContext) getGithubContext() *githubContext {
 		}
 	}
 
-	_, sha, err := common.FindGitRevision(repoPath)
-	if err != nil {
-		log.Warningf("unable to get git revision: %v", err)
-	} else {
-		ghc.Sha = sha
-	}
-
 	if rc.EventJSON != "" {
 		err = json.Unmarshal([]byte(rc.EventJSON), &ghc.Event)
 		if err != nil {
 			log.Errorf("Unable to Unmarshal event '%s': %v", rc.EventJSON, err)
 		}
 	}
 
-	maybeRef := nestedMapLookup(ghc.Event, ghc.EventName, "ref")
-	if maybeRef != nil {
-		log.Debugf("using github ref from event: %s", maybeRef)
-		ghc.Ref = maybeRef.(string)
-	} else {
+	if ghc.EventName == "pull_request" {
+		ghc.BaseRef = asString(nestedMapLookup(ghc.Event, "pull_request", "base", "ref"))
+		ghc.HeadRef = asString(nestedMapLookup(ghc.Event, "pull_request", "head", "ref"))
+	}
+
+	ghc.setRefAndSha(rc.Config.DefaultBranch, repoPath)
+
+	return ghc
+}
+
+func (ghc *githubContext) setRefAndSha(defaultBranch string, repoPath string) {
+	// https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows
+	// https://docs.github.com/en/developers/webhooks-and-events/webhooks/webhook-events-and-payloads
+	switch ghc.EventName {
+	case "pull_request_target":
+		ghc.Ref = ghc.BaseRef
+		ghc.Sha = asString(nestedMapLookup(ghc.Event, "pull_request", "base", "sha"))
+	case "pull_request", "pull_request_review", "pull_request_review_comment":
+		ghc.Ref = fmt.Sprintf("refs/pull/%s/merge", ghc.Event["number"])
+	case "deployment", "deployment_status":
+		ghc.Ref = asString(nestedMapLookup(ghc.Event, "deployment", "ref"))
+		ghc.Sha = asString(nestedMapLookup(ghc.Event, "deployment", "sha"))
+	case "release":
+		ghc.Ref = asString(nestedMapLookup(ghc.Event, "release", "tag_name"))
+	case "push", "create", "workflow_dispatch":
+		ghc.Ref = asString(ghc.Event["ref"])
+		if deleted, ok := ghc.Event["deleted"].(bool); ok && !deleted {
+			ghc.Sha = asString(ghc.Event["after"])
+		}
+	default:
+		ghc.Ref = asString(nestedMapLookup(ghc.Event, "repository", "default_branch"))
+	}
+
+	if ghc.Ref == "" {
 		ref, err := common.FindGitRef(repoPath)
 		if err != nil {
 			log.Warningf("unable to get git ref: %v", err)
@@ -604,19 +626,21 @@ func (rc *RunContext) getGithubContext() *githubContext {
 		}
 
 		// set the branch in the event data
-		if rc.Config.DefaultBranch != "" {
-			ghc.Event = withDefaultBranch(rc.Config.DefaultBranch, ghc.Event)
+		if defaultBranch != "" {
+			ghc.Event = withDefaultBranch(defaultBranch, ghc.Event)
 		} else {
 			ghc.Event = withDefaultBranch("master", ghc.Event)
 		}
 	}
 
-	if ghc.EventName == "pull_request" {
-		ghc.BaseRef = asString(nestedMapLookup(ghc.Event, "pull_request", "base", "ref"))
-		ghc.HeadRef = asString(nestedMapLookup(ghc.Event, "pull_request", "head", "ref"))
+	if ghc.Sha == "" {
+		_, sha, err := common.FindGitRevision(repoPath)
+		if err != nil {
+			log.Warningf("unable to get git revision: %v", err)
+		} else {
+			ghc.Sha = sha
+		}
 	}
-
-	return ghc
 }
 
 func (ghc *githubContext) isLocalCheckout(step *model.Step) bool {