sketch/loop: fuss more with /git/cat and 204s

For unknown reasons, switching to 204s made the diff view noticeably slower.
Be more explicit about things in the hope that it helps.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s52c5ab6557e1f9dbk
diff --git a/loop/server/loophttp.go b/loop/server/loophttp.go
index 550760c..6fa3aae 100644
--- a/loop/server/loophttp.go
+++ b/loop/server/loophttp.go
@@ -1443,12 +1443,14 @@
 
 	// Get file content using GitCat
 	content, err := git_tools.GitCat(repoDir, path)
-	if errors.Is(err, os.ErrNotExist) {
+	switch {
+	case err == nil:
+		// continued below
+	case errors.Is(err, os.ErrNotExist), strings.Contains(err.Error(), "not tracked by git"):
 		w.WriteHeader(http.StatusNoContent)
 		return
-	}
-	if err != nil {
-		http.Error(w, fmt.Sprintf("Error reading file: %v", err), http.StatusInternalServerError)
+	default:
+		http.Error(w, fmt.Sprintf("error reading file: %v", err), http.StatusInternalServerError)
 		return
 	}