Fix successful return value for `SyncAndGetUserSpecificDiff` (#27152) (#27156)

Backport #27152 by @delvh

A function should not return an error when it is successful.
Otherwise, things like
https://discord.com/channels/322538954119184384/322538954119184384/1153705341620600833
happen…

Co-authored-by: delvh <dev.lh@web.de>

services/gitdiff/gitdiff.go

@@ -1312,7 +1312,7 @@ outer:
 		}
 	}
 
-	return diff, err
+	return diff, nil
 }
 
 // CommentAsDiff returns c.Patch as *Diff