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/webui/src/web-components/git-data-service.ts b/webui/src/web-components/git-data-service.ts
index b3aec25..6ff89af 100644
--- a/webui/src/web-components/git-data-service.ts
+++ b/webui/src/web-components/git-data-service.ts
@@ -166,6 +166,9 @@
           `Failed to fetch working copy content (${response.status}): ${errorText}`,
         );
       }
+      if (response.status === 204) {
+        return ""; // file doesn't exist
+      }
 
       const data = await response.json();
       return data.output || "";