fix: update go-git-fixtures calls to use new signatures with error returns

Co-authored-by: pjbgf <5452977+pjbgf@users.noreply.github.com>

Author: Copilot

cmd/gogit/verify-pack.go

@@ -165,7 +165,17 @@ func openPack(path string) (billy.File, billy.File, error) {
 
 		fixture := f.One()
 
-		return fixture.Idx(), fixture.Packfile(), nil
+		idxFile, err := fixture.Idx()
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to open fixture idx file: %w", err)
+		}
+
+		packFile, err := fixture.Packfile()
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to open fixture packfile: %w", err)
+		}
+
+		return idxFile, packFile, nil
 	}
 
 	idxPath := path