update the state of a few TODOs related to upstream Go

The generics issue has been fixed for the upcoming Go 1.20.
Include that version as a reminder for when we can drop Go 1.19.

The fs.SkipAll proposal is also implemented for Go 1.20.

The BinaryContentID comment was a little bit trickier.
We did get stamped VCS information some time ago,
but it only provides us with the current commit info and a dirty bit.
That is not enough for our use of the build cache,
because we want any uncommitted changes to garble to cause rebuilds.

I don't think we'll get any better than using garble's own build ID.
Reword the quasi-TODO to instead explain what we're doing and why.

internal/literals/obfuscators.go

@@ -21,7 +21,7 @@ var obfuscators = []obfuscator{
 	swap{},
 	split{},
 	shuffle{},
-	// seed{}, TODO: re-enable once https://github.com/golang/go/issues/47631 is fixed
+	// seed{}, TODO: re-enable once https://go.dev/issue/47631 is fixed in Go 1.20
 }
 
 // If math/rand.Seed() is not called, the generator behaves as if seeded by rand.Seed(1),

main_test.go

@@ -288,8 +288,7 @@ func grepfiles(ts *testscript.TestScript, neg bool, args []string) {
 	anyFound := false
 	path, pattern := args[0], args[1]
 	rx := regexp.MustCompile(pattern)
-	// TODO: use https://github.com/golang/go/issues/47209 when merged,
-	// hopefully in Go 1.20.
+	// TODO: use fs.SkipAll in Go 1.20 and later.
 	errSkipAll := fmt.Errorf("sentinel error: stop walking")
 	if err := filepath.WalkDir(path, func(path string, d fs.DirEntry, err error) error {
 		if err != nil {

shared.go

@@ -33,12 +33,12 @@ type sharedCache struct {
 	// useful for type checking of the packages as we obfuscate them.
 	ListedPackages map[string]*listedPackage
 
-	// We can't rely on the module version to exist,
-	// because it's missing in local builds without 'go install'.
-	// For now, use 'go tool buildid' on the garble binary.
-	// Just like Go's own cache, we use hex-encoded sha256 sums.
-	// Once https://github.com/golang/go/issues/37475 is fixed,
-	// we can likely just use that.
+	// We can't use garble's own module version, as it may not exist.
+	// We can't use the stamped VCS information either,
+	// as uncommitted changes simply show up as "dirty".
+	//
+	// The only unique way to identify garble's version without being published
+	// or committed is to use its content ID from the build cache.
 	BinaryContentID []byte
 
 	GOGARBLE string
@@ -228,7 +228,7 @@ func appendListedPackages(packages []string, withDeps bool) error {
 			pkg.GarbleActionID = addGarbleToHash(actionID)
 		}
 
-		// Work around https://golang.org/issue/28749:
+		// Work around https://go.dev/issue/28749:
 		// cmd/go puts assembly, C, and C++ files in CompiledGoFiles.
 		//
 		// TODO: remove when upstream has fixed the bug.