add gist clone command

This adds the ability to clone a gist.

Usage:

```sh
$ gh gist clone 5b0e0062eb8e9654adad7bb1d81cc75f
$ gh gist clone https://gist.github.com/OWNER/5b0e0062eb8e9654adad7bb1d81cc75f
```

This closes #2115.

pkg/cmd/gist/clone/clone.go

@@ -0,0 +1,101 @@
+package clone
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/MakeNowJust/heredoc"
+	"github.com/cli/cli/git"
+	"github.com/cli/cli/internal/config"
+	"github.com/cli/cli/internal/ghinstance"
+	"github.com/cli/cli/pkg/cmdutil"
+	"github.com/cli/cli/pkg/iostreams"
+	"github.com/spf13/cobra"
+	"github.com/spf13/pflag"
+)
+
+type CloneOptions struct {
+	HttpClient func() (*http.Client, error)
+	Config     func() (config.Config, error)
+	IO         *iostreams.IOStreams
+
+	GitArgs   []string
+	Directory string
+	Gist      string
+}
+
+func NewCmdClone(f *cmdutil.Factory, runF func(*CloneOptions) error) *cobra.Command {
+	opts := &CloneOptions{
+		IO:         f.IOStreams,
+		HttpClient: f.HttpClient,
+		Config:     f.Config,
+	}
+
+	cmd := &cobra.Command{
+		DisableFlagsInUseLine: true,
+
+		Use:   "clone <gist> [<directory>] [-- <gitflags>...]",
+		Args:  cmdutil.MinimumArgs(1, "cannot clone: gist argument required"),
+		Short: "Clone a gist locally",
+		Long: heredoc.Doc(`
+			Clone a GitHub gist locally.
+
+			A gist can be supplied as argument in either of the following formats:
+			- by ID, e.g. 5b0e0062eb8e9654adad7bb1d81cc75f
+			- by URL, e.g. "https://gist.github.com/OWNER/5b0e0062eb8e9654adad7bb1d81cc75f"
+
+			Pass additional 'git clone' flags by listing them after '--'.
+		`),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			opts.Gist = args[0]
+			opts.GitArgs = args[1:]
+
+			if runF != nil {
+				return runF(opts)
+			}
+
+			return cloneRun(opts)
+		},
+	}
+
+	cmd.SetFlagErrorFunc(func(cmd *cobra.Command, err error) error {
+		if err == pflag.ErrHelp {
+			return err
+		}
+		return &cmdutil.FlagError{Err: fmt.Errorf("%w\nSeparate git clone flags with '--'.", err)}
+	})
+
+	return cmd
+}
+
+func cloneRun(opts *CloneOptions) error {
+	gistURL := opts.Gist
+
+	if !git.IsURL(gistURL) {
+		cfg, err := opts.Config()
+		if err != nil {
+			return err
+		}
+		hostname := ghinstance.OverridableDefault()
+		protocol, err := cfg.Get(hostname, "git_protocol")
+		if err != nil {
+			return err
+		}
+		gistURL = formatRemoteURL(hostname, gistURL, protocol)
+	}
+
+	_, err := git.RunClone(gistURL, opts.GitArgs)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func formatRemoteURL(hostname string, gistID string, protocol string) string {
+	if protocol == "ssh" {
+		return fmt.Sprintf("git@gist.%s:%s.git", hostname, gistID)
+	}
+
+	return fmt.Sprintf("https://gist.%s/%s.git", hostname, gistID)
+}

pkg/cmd/gist/clone/clone_test.go

@@ -0,0 +1,118 @@
+package clone
+
+import (
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/cli/cli/internal/config"
+	"github.com/cli/cli/pkg/cmdutil"
+	"github.com/cli/cli/pkg/httpmock"
+	"github.com/cli/cli/pkg/iostreams"
+	"github.com/cli/cli/test"
+	"github.com/google/shlex"
+	"github.com/stretchr/testify/assert"
+)
+
+func runCloneCommand(httpClient *http.Client, cli string) (*test.CmdOut, error) {
+	io, stdin, stdout, stderr := iostreams.Test()
+	fac := &cmdutil.Factory{
+		IOStreams: io,
+		HttpClient: func() (*http.Client, error) {
+			return httpClient, nil
+		},
+		Config: func() (config.Config, error) {
+			return config.NewBlankConfig(), nil
+		},
+	}
+
+	cmd := NewCmdClone(fac, nil)
+
+	argv, err := shlex.Split(cli)
+	cmd.SetArgs(argv)
+
+	cmd.SetIn(stdin)
+	cmd.SetOut(stdout)
+	cmd.SetErr(stderr)
+
+	if err != nil {
+		panic(err)
+	}
+
+	_, err = cmd.ExecuteC()
+
+	if err != nil {
+		return nil, err
+	}
+
+	return &test.CmdOut{OutBuf: stdout, ErrBuf: stderr}, nil
+}
+
+func Test_GistClone(t *testing.T) {
+	tests := []struct {
+		name string
+		args string
+		want string
+	}{
+		{
+			name: "shorthand",
+			args: "GIST",
+			want: "git clone https://gist.github.com/GIST.git",
+		},
+		{
+			name: "shorthand with directory",
+			args: "GIST target_directory",
+			want: "git clone https://gist.github.com/GIST.git target_directory",
+		},
+		{
+			name: "clone arguments",
+			args: "GIST -- -o upstream --depth 1",
+			want: "git clone -o upstream --depth 1 https://gist.github.com/GIST.git",
+		},
+		{
+			name: "clone arguments with directory",
+			args: "GIST target_directory -- -o upstream --depth 1",
+			want: "git clone -o upstream --depth 1 https://gist.github.com/GIST.git target_directory",
+		},
+		{
+			name: "HTTPS URL",
+			args: "https://gist.github.com/OWNER/GIST",
+			want: "git clone https://gist.github.com/OWNER/GIST",
+		},
+		{
+			name: "SSH URL",
+			args: "git@gist.github.com:GIST.git",
+			want: "git clone git@gist.github.com:GIST.git",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			reg := &httpmock.Registry{}
+
+			httpClient := &http.Client{Transport: reg}
+
+			cs, restore := test.InitCmdStubber()
+			defer restore()
+
+			cs.Stub("") // git clone
+
+			output, err := runCloneCommand(httpClient, tt.args)
+			if err != nil {
+				t.Fatalf("error running command `gist clone`: %v", err)
+			}
+
+			assert.Equal(t, "", output.String())
+			assert.Equal(t, "", output.Stderr())
+			assert.Equal(t, 1, cs.Count)
+			assert.Equal(t, tt.want, strings.Join(cs.Calls[0].Args, " "))
+			reg.Verify(t)
+		})
+	}
+}
+
+func Test_GistClone_flagError(t *testing.T) {
+	_, err := runCloneCommand(nil, "--depth 1 GIST")
+	if err == nil || err.Error() != "unknown flag: --depth\nSeparate git clone flags with '--'." {
+		t.Errorf("unexpected error %v", err)
+	}
+}

pkg/cmd/gist/gist.go

@@ -2,6 +2,7 @@ package gist
 
 import (
 	"github.com/MakeNowJust/heredoc"
+	gistCloneCmd "github.com/cli/cli/pkg/cmd/gist/clone"
 	gistCreateCmd "github.com/cli/cli/pkg/cmd/gist/create"
 	gistDeleteCmd "github.com/cli/cli/pkg/cmd/gist/delete"
 	gistEditCmd "github.com/cli/cli/pkg/cmd/gist/edit"
@@ -26,6 +27,7 @@ func NewCmdGist(f *cmdutil.Factory) *cobra.Command {
 		},
 	}
 
+	cmd.AddCommand(gistCloneCmd.NewCmdClone(f, nil))
 	cmd.AddCommand(gistCreateCmd.NewCmdCreate(f, nil))
 	cmd.AddCommand(gistListCmd.NewCmdList(f, nil))
 	cmd.AddCommand(gistViewCmd.NewCmdView(f, nil))