cmd/go/internal/modfetch: do not trust server to send all tags in shallow fetch

Newer git versions (at least git 2.47.1) do not send all the matching tags
for a shallow fetch of a specific hash anymore. The go command assumes
that git servers do this. Since that assumption is broken, use the local
copy of the remote refs list to augment the tags sent by the server.
This makes the cmd/go/internal/modfetch tests pass again with newer git.

Fixes golang#71261.

Change-Id: I9fd4f3fd7beeb68a522938599f8f3acd887d0b26
Reviewed-on: https://go-review.googlesource.com/c/go/+/642437
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Russ Cox <[email protected]>

Author: rsc

src/cmd/go/internal/modfetch/codehost/git.go

@@ -649,7 +649,21 @@ func (r *gitRepo) statLocal(ctx context.Context, version, rev string) (*RevInfo,
 			}
 		}
 	}
-	sort.Strings(info.Tags)
+
+	// Git 2.47.1 does not send the tags during shallow clone anymore
+	// (perhaps the exact version that changed behavior is an earlier one),
+	// so we have to also add tags from the refs list we fetched with ls-remote.
+	if refs, err := r.loadRefs(ctx); err == nil {
+		for ref, h := range refs {
+			if h == hash {
+				if tag, found := strings.CutPrefix(ref, "refs/tags/"); found {
+					info.Tags = append(info.Tags, tag)
+				}
+			}
+		}
+	}
+	slices.Sort(info.Tags)
+	info.Tags = slices.Compact(info.Tags)
 
 	// Used hash as info.Version above.
 	// Use caller's suggested version if it appears in the tag list