R1:d38e89ef6ba5

Fix several issues with application interactions while importing commits

Summary:
- Fixes T5851. Currently, if a commit has `Fixes T123`, we generate an email with just that before generating the commit email. Don't send/publish transactions about a commit before it imports (this is a tiny bit hacky, but well-contained and I don't think it causes any problems).
- Fixes T4864. Currently, we try to parse Differential information even if Differential is not installed. Instead, do this only if…