Check git blob size against cumulative limit before reading it.

This avoids exhausting RAM when reading e.g. a repository with a single
extremely large file. Note that there is still a risk of exhausting
space in `/tmp`.

V12-Ref: F-77211

src/fetch.go

@@ -214,6 +214,14 @@ func readGitBlob(
 		return fmt.Errorf("git blob %s: %w", hash, err)
 	}
 
+	*bytesTransferred += blob.Size
+	if uint64(*bytesTransferred) > config.Limits.MaxSiteSize.Bytes() {
+		return fmt.Errorf("%w: fetch exceeds %s limit",
+			ErrRepositoryTooLarge,
+			config.Limits.MaxSiteSize.HR(),
+		)
+	}
+
 	reader, err := blob.Reader()
 	if err != nil {
 		return fmt.Errorf("git blob open: %w", err)
@@ -238,14 +246,6 @@ func readGitBlob(
 	entry.OriginalSize = proto.Int64(blob.Size)
 	entry.CompressedSize = proto.Int64(blob.Size)
 
-	*bytesTransferred += blob.Size
-	if uint64(*bytesTransferred) > config.Limits.MaxSiteSize.Bytes() {
-		return fmt.Errorf("%w: fetch exceeds %s limit",
-			ErrRepositoryTooLarge,
-			config.Limits.MaxSiteSize.HR(),
-		)
-	}
-
 	return nil
 }