R1:4b3c384856da

Fix DifferentialCommitMessageField renderFieldValue PHP 8.1 strlen(null) error

Summary:
arc diff throws strlen(null) error from DifferentialCommitMessageField renderFieldValue when calling a Phorge server running PHP 8.1

Add unit test, which required a new DifferentialTestCommitMessageField class so as to be able to test the abstract DifferentialCommitMessageField class methods.

Fixes T15530

Test Plan:
Make a change in a git repo with remote a Phorge server running PHP 8.1
Run:

```
arc diff
```
See…