Use existing build information for version if not embedded with build script

Go1.12 introduced BuildInfo which embeds build information. It does
not embed build date to facilitate reproducable builds by default.
If build information is embedded from build script, use the information
provided by the Go build system.

init_dir.go

@@ -55,6 +55,8 @@ func isDir(dir string) error {
 // In reverse mode, we create .gocryptfs.reverse.conf and the directory does
 // not need to be empty.
 func initDir(args *argContainer) {
+	initGIT()
+
 	var err error
 	if args.reverse {
 		_, err = os.Stat(args.config)

main.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"runtime/debug"
 	"strconv"
 	"strings"
 
@@ -36,6 +37,51 @@ var BuildDate = "0000-00-00"
 // raceDetector is set to true by race.go if we are compiled with "go build -race"
 var raceDetector bool
 
+func initGIT() {
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		return
+	}
+	if strings.HasPrefix(GitVersion, `[`) {
+		GitVersion = bi.Main.Version
+		var gitDate string
+		var gitDirty bool
+		for _, item := range bi.Settings {
+			switch item.Key {
+			case "vcs.revision":
+				GitVersion = item.Value
+			case "vcs.time":
+				gitDate = item.Value
+				gitDate = strings.Map(func(r rune) rune {
+					switch {
+					case r >= '0' && r <= '9':
+						return r
+					default:
+						return -1
+					}
+				}, gitDate)
+			case "vcs.modified":
+				gitDirty, _ = strconv.ParseBool(item.Value)
+			}
+		}
+		if len(gitDate) > 0 {
+			GitVersion = gitDate + "-" + GitVersion
+		}
+		if gitDirty {
+			GitVersion = GitVersion + " (dirty)"
+		}
+	}
+	const fuseModule = `github.com/hanwen/go-fuse/v2`
+
+	if strings.HasPrefix(GitVersionFuse, `[`) {
+		for _, item := range bi.Deps {
+			if item.Path == fuseModule {
+				GitVersionFuse = item.Version
+			}
+		}
+	}
+}
+
 // loadConfig loads the config file `args.config` and decrypts the masterkey,
 // or gets via the `-masterkey` or `-zerokey` command line options, if specified.
 func loadConfig(args *argContainer) (masterkey []byte, cf *configfile.ConfFile, err error) {
@@ -140,6 +186,8 @@ func changePassword(args *argContainer) {
 // printVersion prints a version string like this:
 // gocryptfs v1.7-32-gcf99cfd; go-fuse v1.0.0-174-g22a9cb9; 2019-05-12 go1.12 linux/amd64
 func printVersion() {
+	initGIT()
+
 	var tagsSlice []string
 	if stupidgcm.BuiltWithoutOpenssl {
 		tagsSlice = append(tagsSlice, "without_openssl")