Update PR when labeled/opened

This will conditionally update a PR whenever the PR is labeled or opened, which
will partially fulfill [this
suggestion](palantir#145 (comment))
for palantir#145.

It doesn't seem possible to determine which label was added via the
Github webhook payload, so this will update the PR regardless of whether
the label added is what is configured in `bulldozer.yml`.

server/handler/pull_request.go

@@ -17,6 +17,7 @@ package handler
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 
 	"github.com/google/go-github/v32/github"
 	"github.com/palantir/go-githubapp/githubapp"
@@ -66,6 +67,21 @@ func (h *PullRequest) Handle(ctx context.Context, eventType, deliveryID string,
 		logger.Error().Err(errors.WithStack(err)).Msg("Error processing pull request")
 	}
 
+	if event.GetAction() == "labeled" || event.GetAction() == "opened" {
+		logger.Debug().Msgf("received pull_request %s event", event.GetAction())
+
+		base, _ := pullCtx.Branches()
+		ref, _, err := client.Git.GetRef(ctx, pullCtx.Owner(), pullCtx.Repo(), fmt.Sprintf("refs/heads/%s", base))
+		if err != nil {
+			return errors.Wrap(err, "could not get git reference of PR base branch")
+		}
+
+		baseRef := ref.GetObject().GetSHA()
+		if err := h.UpdatePullRequest(logger.WithContext(ctx), pullCtx, client, pr, baseRef); err != nil {
+			logger.Error().Err(errors.WithStack(err)).Msg("Error updating pull request")
+		}
+	}
+
 	return nil
 }