Add pr update command

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>

pkg/cmd/pr/update/update.go

@@ -0,0 +1,120 @@
+package update
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/MakeNowJust/heredoc"
+	"github.com/cli/cli/v2/api"
+	"github.com/cli/cli/v2/git"
+	"github.com/cli/cli/v2/internal/ghrepo"
+	shared "github.com/cli/cli/v2/pkg/cmd/pr/shared"
+	"github.com/cli/cli/v2/pkg/cmdutil"
+	"github.com/cli/cli/v2/pkg/iostreams"
+	"github.com/shurcooL/githubv4"
+	"github.com/spf13/cobra"
+)
+
+type UpdateOptions struct {
+	HttpClient func() (*http.Client, error)
+	IO         *iostreams.IOStreams
+	GitClient  *git.Client
+
+	Finder shared.PRFinder
+
+	SelectorArg string
+	Rebase      bool
+}
+
+func NewCmdUpdate(f *cmdutil.Factory, runF func(*UpdateOptions) error) *cobra.Command {
+	opts := &UpdateOptions{
+		IO:         f.IOStreams,
+		HttpClient: f.HttpClient,
+		GitClient:  f.GitClient,
+	}
+
+	cmd := &cobra.Command{
+		Use:   "update [<number> | <url> | <branch>]",
+		Short: "Update a pull request branch",
+		Long: heredoc.Docf(`
+			Update a pull request branch with latest changes of the base branch.
+
+			Without an argument, the pull request that belongs to the current branch is selected.
+
+			The default behavior is to update with a merge commit (i.e., merging the base branch
+			into the the PR's branch). To reconcile the changes with rebasing on top of the base
+			branch, the %[1]s--rebase%[1]s option should be provided.
+		`, "`"),
+		Example: heredoc.Doc(`
+			$ gh pr update 23
+			$ gh pr update 23 --rebase
+		`),
+		Args: cobra.MaximumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			opts.Finder = shared.NewFinder(f)
+
+			if len(args) > 0 {
+				opts.SelectorArg = args[0]
+			}
+
+			if runF != nil {
+				return runF(opts)
+			}
+
+			return updateRun(opts)
+		},
+	}
+
+	cmd.Flags().BoolVar(&opts.Rebase, "rebase", false, "Update PR branch by rebasing on top of latest base branch")
+
+	_ = cmdutil.RegisterBranchCompletionFlags(f.GitClient, cmd, "base")
+
+	return cmd
+}
+
+func updateRun(opts *UpdateOptions) error {
+	findOptions := shared.FindOptions{
+		Selector: opts.SelectorArg,
+		Fields:   []string{"id", "headRefOid"},
+	}
+
+	pr, repo, err := opts.Finder.Find(findOptions)
+	if err != nil {
+		return err
+	}
+
+	httpClient, err := opts.HttpClient()
+	if err != nil {
+		return err
+	}
+	apiClient := api.NewClientFromHTTP(httpClient)
+
+	opts.IO.StartProgressIndicator()
+	err = updatePullRequestBranch(apiClient, repo, pr.ID, pr.HeadRefOid, opts.Rebase)
+	opts.IO.StopProgressIndicator()
+
+	if err != nil {
+		return err
+	}
+
+	cs := opts.IO.ColorScheme()
+	fmt.Fprintf(opts.IO.ErrOut, "%s PR branch updated\n", cs.SuccessIcon())
+	return nil
+}
+
+// updatePullRequestBranch calls the GraphQL API endpoint to update the given PR
+// branch with latest changes of its base.
+func updatePullRequestBranch(apiClient *api.Client, repo ghrepo.Interface, pullRequestID string, expectedHeadOid string, rebase bool) error {
+	updateMethod := githubv4.PullRequestBranchUpdateMethodMerge
+	if rebase {
+		updateMethod = githubv4.PullRequestBranchUpdateMethodRebase
+	}
+
+	params := githubv4.UpdatePullRequestBranchInput{
+		PullRequestID:   pullRequestID,
+		ExpectedHeadOid: (*githubv4.GitObjectID)(&expectedHeadOid),
+		UpdateMethod:    &updateMethod,
+	}
+
+	return api.UpdatePullRequestBranch(apiClient, repo, params)
+}