fix: improve git diff --no-index error handling

Check exitCode === 1 instead of message content, as git diff --no-index
returns exit code 1 when differences exist (which is expected behavior,
not an error).

Amp-Thread-ID: https://ampcode.com/threads/T-019c0a9a-b26b-7719-9718-04185371eb9c
Co-authored-by: Amp <amp@ampcode.com>

packages/web/server/lib/git-service.js

@@ -428,7 +428,8 @@ export async function getDiff(directory, { path, staged = false, contextLines =
         const noIndexDiff = await git.raw(noIndexArgs);
         return noIndexDiff;
       } catch (noIndexError) {
-        if (noIndexError.message && noIndexError.message.includes('diff --git')) {
+        // git diff --no-index returns exit code 1 when differences exist (not a real error)
+        if (noIndexError.exitCode === 1 && noIndexError.message) {
           return noIndexError.message;
         }
         throw noIndexError;