Expose version via `--version` and `User-Agent:`.

The version is sent via `User-Agent:` because it may be useful at
some point to work around bugs server-side (or prompt the user to
update). If undesired, build with `-buildvcs=false`.

Packagers building without a full git checkout should override version
in the `version.go` file.

main.go

···
 	"net/http"
 	"net/url"
 	"os"
+	"runtime/debug"
 
 	"github.com/google/uuid"
 	"github.com/klauspost/compress/zstd"
···
 var debugManifestFlag = pflag.Bool("debug-manifest", false, "retrieve site manifest as ProtoJSON, for debugging")
 var serverFlag = pflag.String("server", "", "hostname of server to connect to")
 var verboseFlag = pflag.Bool("verbose", false, "display more information for debugging")
+var versionFlag = pflag.Bool("version", false, "display version information")
 
 func singleOperation() bool {
 	operations := 0
···
 	if *debugManifestFlag {
 		operations++
 	}
+	if *versionFlag {
+		operations++
+	}
 	return operations == 1
 }
 
+func versionInfo() string {
+	version := "(unknown)"
+	if versionOverride != "" {
+		version = versionOverride
+	} else if buildInfo, ok := debug.ReadBuildInfo(); ok {
+		version = buildInfo.Main.Version
+	}
+	return fmt.Sprintf("git-pages-cli %s", version)
+}
+
 func displayFS(root fs.FS) error {
 	return fs.WalkDir(root, ".", func(name string, entry fs.DirEntry, err error) error {
 		if err != nil {
···
 
 func main() {
 	pflag.Parse()
-	if !singleOperation() || len(pflag.Args()) != 1 {
+	if !singleOperation() || (!*versionFlag && len(pflag.Args()) != 1) {
 		fmt.Fprintf(os.Stderr,
 			"Usage: %s <site-url> [--challenge|--upload-git url|--upload-dir path|--delete]\n",
 			os.Args[0],
 		)
 		os.Exit(125)
+	}
+
+	if *versionFlag {
+		fmt.Fprintln(os.Stdout, versionInfo())
+		os.Exit(0)
 	}
 
 	var err error
···
 	default:
 		panic("no operation chosen")
 	}
+	request.Header.Add("User-Agent", versionInfo())
 	if *passwordFlag != "" {
 		request.Header.Add("Authorization", fmt.Sprintf("Pages %s", *passwordFlag))
 	}

version.go

···
+// By default the version information is retrieved from VCS.
+// If not available during build, override it here:
+
+package main
+
+const versionOverride = "" // set to e.g. "v1.2.3" as appropriate