R1:f81e821abf2b

Mark manually created Diffs as new objects to fix empty Feed transaction entries

Summary:
When creating a Differential diff manually via `/differential/diff/create/` in the web browser instead of using Arcanist, `setIsCreateTransaction(true)` for the transaction type `DifferentialDiffTransaction::TYPE_DIFF_CREATE` to avoid an empty `Transaction` field in the Feed and avoid `strncmp()` complaining about a null value being passed.

For gory details, see the comments in T15659.

```
ERROR 8192: strncmp()…