internal/stdlib: fix getGoRepo for requests to master

getGoRepo currently looks for a "master" tag when fetching the master
repo, instead of at the master branch. This is now fixed.

However, stdlib.semanticVersion still returns "master" when the
requestedVersion is "master", instead of returning a resolved semantic
version, so a fetch request for std@master will still result in a 500. A
comment is added to fix this in the future.

In a future CL, a fake pseudoversion will need to be constructed in
order to resolve std@master to a semantic version.

For golang/go#43890

Change-Id: I9e68b91d8c9e0bea6f6751d834853b852f0d532b
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/286153
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Trust: Julie Qiu <julie@golang.org>

internal/stdlib/stdlib.go

@@ -180,13 +180,19 @@ var TestCommitTime = time.Date(2019, 9, 4, 1, 2, 3, 0, time.UTC)
 func getGoRepo(version string) (_ *git.Repository, err error) {
 	defer derrors.Wrap(&err, "getGoRepo(%q)", version)
 
-	tag, err := TagForVersion(version)
-	if err != nil {
-		return nil, err
+	var ref plumbing.ReferenceName
+	if version == "master" {
+		ref = plumbing.HEAD
+	} else {
+		tag, err := TagForVersion(version)
+		if err != nil {
+			return nil, err
+		}
+		ref = plumbing.NewTagReferenceName(tag)
 	}
 	return git.Clone(memory.NewStorage(), nil, &git.CloneOptions{
 		URL:           GoRepoURL,
-		ReferenceName: plumbing.NewTagReferenceName(tag),
+		ReferenceName: ref,
 		SingleBranch:  true,
 		Depth:         1,
 		Tags:          git.NoTags,
@@ -375,6 +381,8 @@ func semanticVersion(requestedVersion string) (_ string, err error) {
 		}
 		return latestVersion, nil
 	case "master":
+		// TODO(https://github.com/golang/go/issues/43890): a semantic version
+		// needs to be returned here.
 		return requestedVersion, nil
 	default:
 		for _, v := range knownVersions {