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(): Passing null to parameter #1 ($string1) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/policy/query/PhabricatorPolicyQuery.php:314]

Closes T15659

Test Plan:

1. Create a valid diff via /differential/diff/create/
2. Go to /feed/transactions/query/advanced/ and set Object Types to Differential Diff and click Search
3. See two entries in the Transaction column for the just created diff: One says "created this diff", the other one says "created this object with visibility" and is not an empty line anymore which triggered an exception.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15659

Differential Revision: https://we.phorge.it/D25517